avcodec/mpeg4videodec: Fix nonsense warning

avcodec/mpeg4videodec: Fix nonsense warning

Since db772308941a2a338c7809f90d347219a6a93074 parsing of
mpeg4-extradata lead to a "Failed to parse extradata" warning, because
ff_mpeg4_decode_picture_header returns AVERROR_INVALIDDATA in case that
no VOP was found. This patch adds a parameter to signify whether a
header (where the absence of a VOP does not raise an error) or not is
parsed. The first mode is of course used for parsing headers.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>