avcodec/vp7: Fix null pointer dereference in vp7_decode_frame_header()

This simply copies the "interframe without a prior keyframe" check
thats done later into vp7_decode_frame_header()

Found-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
