fix #121: null pointer dereference at cli/wvunpack.c

[xiaoxiaoafeifei]

fix #121: null pointer dereference at cli/wvunpack.c:911

[dbry]

Hi. I have pushed a more involved fix for this. Your PR would fix the test case, but fails if you add another instance of the file like this:

/usr/local/bin/wvunpack -m poc.wv poc.wv -o /

I also decided to sanitize the custom extensions from the library to be even safer.

Thanks!