net: fix crash of Listen with "" or nil laddr

Fixes issue 3584.

R=dave, dsymonds, rsc
CC=golang-dev
http://codereview.appspot.com/6119043

http://code.google.com/p/go/source/detail?r=b07591eb8077

$ tail -200 < log
crypto/ecdsa
crypto/sha256
crypto/sha512
database/sql/driver
database/sql
debug/gosym
encoding/ascii85
encoding/base32
encoding/csv
encoding/hex
exp/ebnf
exp/ebnflint
exp/types
exp/gotype
exp/html
exp/norm
exp/proxy
exp/utf8string
exp/winfsnotify
hash/crc64
hash/fnv
image/color
image
image/draw
image/gif
image/jpeg
image/png
log/syslog
math/cmplx
net/http/cgi
net/http/fcgi
net/http/httptest
net/http/httputil
net/mail
net/rpc
net/rpc/jsonrpc
net/smtp
old/netchan
old/regexp
old/template
os/signal
os/user
runtime/cgo
testing
testing/iotest
testing/quick

# Testing packages.
ok      cmd/api 0.047s
?       cmd/cgo [no test files]
ok      cmd/fix 1.498s
ok      cmd/go  0.051s
?       cmd/godoc       [no test files]
ok      cmd/gofmt       0.072s
?       cmd/vet [no test files]
?       cmd/yacc        [no test files]
ok      archive/tar     0.026s
ok      archive/zip     0.069s
ok      bufio   0.093s
ok      bytes   0.064s
ok      compress/bzip2  0.097s
ok      compress/flate  0.201s
ok      compress/gzip   0.036s
ok      compress/lzw    0.043s
ok      compress/zlib   0.154s
ok      container/heap  0.025s
ok      container/list  0.024s
ok      container/ring  0.080s
?       crypto  [no test files]
ok      crypto/aes      0.055s
ok      crypto/cipher   0.089s
ok      crypto/des      0.048s
ok      crypto/dsa      0.038s
ok      crypto/ecdsa    0.092s
ok      crypto/elliptic 0.039s
ok      crypto/hmac     0.026s
ok      crypto/md5      0.025s
ok      crypto/rand     0.048s
ok      crypto/rc4      0.025s
ok      crypto/rsa      0.151s
ok      crypto/sha1     0.027s
ok      crypto/sha256   0.131s
ok      crypto/sha512   0.027s
ok      crypto/subtle   0.025s
ok      crypto/tls      0.161s
ok      crypto/x509     0.538s
?       crypto/x509/pkix        [no test files]
ok      database/sql    0.034s
ok      database/sql/driver     0.025s
ok      debug/dwarf     0.032s
ok      debug/elf       0.040s
ok      debug/gosym     0.026s
ok      debug/macho     0.028s
ok      debug/pe        0.029s
ok      encoding/ascii85        0.019s
ok      encoding/asn1   0.031s
ok      encoding/base32 0.026s
ok      encoding/base64 0.019s
ok      encoding/binary 0.027s
ok      encoding/csv    0.026s
ok      encoding/gob    0.068s
ok      encoding/hex    0.020s
ok      encoding/json   0.080s
ok      encoding/pem    0.024s
ok      encoding/xml    0.042s
ok      errors  0.025s
ok      exp/ebnf        0.025s
ok      exp/ebnflint    0.030s
ok      exp/gotype      0.233s
ok      exp/html        0.165s
ok      exp/norm        1.458s
ok      exp/proxy       0.028s
ok      exp/types       0.035s
ok      exp/utf8string  0.027s
ok      exp/winfsnotify 0.080s
ok      expvar  0.030s
ok      flag    0.024s
ok      fmt     0.076s
ok      go/ast  0.031s
ok      go/build        0.295s
ok      go/doc  0.107s
ok      go/parser       0.075s
ok      go/printer      0.384s
ok      go/scanner      0.029s
ok      go/token        0.074s
?       hash    [no test files]
ok      hash/adler32    0.024s
ok      hash/crc32      0.024s
ok      hash/crc64      0.024s
ok      hash/fnv        0.025s
ok      html    0.026s
ok      html/template   0.085s
ok      image   0.143s
ok      image/color     0.037s
ok      image/draw      0.063s
?       image/gif       [no test files]
ok      image/jpeg      0.121s
ok      image/png       0.089s
ok      index/suffixarray       0.033s
ok      io      0.039s
ok      io/ioutil       0.029s
ok      log     0.026s
?       log/syslog      [no test files]
ok      math    0.028s
ok      math/big        0.162s
ok      math/cmplx      0.026s
ok      math/rand       0.206s
ok      mime    0.040s
ok      mime/multipart  0.201s
ok      net     0.967s
--- FAIL: TestServerExpect (0.01 seconds)
        serve_test.go:691: ReadString: WSARecv tcp 127.0.0.1:65126: An existing  
connection was forcibly closed by the remote host.
FAIL
FAIL    net/http        8.413s
ok      net/http/cgi    0.085s
ok      net/http/fcgi   0.038s
ok      net/http/httptest       0.040s
ok      net/http/httputil       0.044s
?       net/http/pprof  [no test files]
ok      net/mail        0.029s
ok      net/rpc 0.100s
ok      net/rpc/jsonrpc 0.047s
ok      net/smtp        0.034s
ok      net/textproto   0.028s
ok      net/url 0.020s
ok      old/netchan     0.042s
ok      old/regexp      0.040s
ok      old/template    0.088s
ok      os      0.098s
ok      os/exec 0.140s
ok      os/signal       0.025s
ok      os/user 22.602s
ok      path    0.033s
ok      path/filepath   0.041s
ok      reflect 0.030s
ok      regexp  0.115s
ok      regexp/syntax   0.961s
ok      runtime 1.543s
?       runtime/cgo     [no test files]
ok      runtime/debug   0.026s
ok      runtime/pprof   0.401s
ok      sort    0.126s
ok      strconv 0.629s
ok      strings 0.030s
ok      sync    0.078s
ok      sync/atomic     0.033s
?       syscall [no test files]
?       testing [no test files]
?       testing/iotest  [no test files]
ok      testing/quick   0.061s
ok      text/scanner    0.028s
ok      text/tabwriter  0.025s
ok      text/template   0.053s
ok      text/template/parse     0.031s
ok      time    41.024s
ok      unicode 0.027s
ok      unicode/utf16   0.027s
ok      unicode/utf8    0.030s
?       unsafe  [no test files]