search for real extradata start instead of always skipping one byte.

Patch by Andrew Dennison gmailify(${name}d, lists)
Thread: [PATCH] Fix VC1 "Incomplete extradata" for mkv files generated by eac3to



git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20178 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b