[kanji.sljfaq.org/kanjivg.html] Update to deal with bad requests

[Ben Bullock]

A couple of days ago, somebody sent several hundred requests for PROPFIND to the underlying software for the kanji stroke order diagrams page, which is here:

http://kanji.sljfaq.org/kanjivg/memory.cgi

It's "memory.cgi" because the image is made in memory. Anyway this caused a huge list of errors in the log, so I've spent an hour or so making it robust against these requests. I didn't update the kanjivg data.