bstr.start can be NULL when bstr.len is 0, so don't call memcmp or
strncasecmp if that's the case. Passing NULL to string functions is
invalid C, even when the length is 0, and it causes Windows to raise an
invalid parameter error.
Should fix#1155
bstr.c doesn't really deserve its own directory, and compat had just
a few files, most of which may as well be in osdep. There isn't really
any justification for these extra directories, so get rid of them.
The compat/libav.h was empty - just delete it. We changed our approach
to API compatibility, and will likely not need it anymore.
