potential bugs in "remote_stdio"

[Qian Yun]

First, the buffer "buf" is not reset to '\0's in
each loop, this could cause problem if we want to
support multi-line input someday.

Second, if "session" sends "str1\0" and "str2\0"
consecutively, then "remote_stdio" might get "str1\0str2\0"
as a whole and only print out the fist part.
This can explain a part of the "lines missing when using
fricas through a pipe" bug.

- Qian