BSTR_P() passes the string length and start pointer to printf-like
functions. If the lenfth is 0, the pointer can be NULL, but we're
actually still not allowed to pass a NULL pointer in any case.
This is mostly a technically, because nobody in their right mind would
attempt to specifically break such cases. But it's still undefined
behavior, and some libcs might be strict about this.
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.
