Add fixes to sceMpegAvcDecodeYCbCr().

It seems a lot like sceMpegAvcDecode(), these are just copy/paste.
Allow a video end a couple packets from end.

Fixes #2206, Patapon 3.  Possibly we don't even need this check at all,
but I'm not sure.
Merge pull request #2207 from unknownbrackets/mpeg-fix

Allow an mpeg to finish even not completely at stream end