fix windows crash and probably other random nastiness.

fix windows crash and probably other random nastiness.
Initializing extradata_size to non-zero makes ffmpeg think it
needs to parse extradata.  But in the non-broken-by-microsoft case
we leave extradata as uninitialized junk.  So ffmpeg parses uninitialized
data and sometimes marches off into the weeds.

git-svn-id: svn://localhost/HandBrake/trunk@3651 b64f7644-9d1e-0410-96f1-a4d463321fa5