Fix uninitialized variable in libav extractor.
Fix uninitialized variable in libav extractor.

And warning about passing NULL to an argument that takes an int in
vorbis extractor.

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker