h264: improve checks before calling ff_h264_decode_extradata

  The ff_h264_decode_extradata routine now checks for the buffer size
and pointer internally. This makes it possible to remove the external
checks in ff_h264_decode_init.

  In decode_frame there was a size check missing because the buffer
gets tested prior to the invocation of ff_h264_decode_extradata().

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
