The frame is not needed that early so obtaining it later avoids
the costly operation in case other checks fail.

Fixes: Timeout (14sec -> 4sec)
Fixes: 13140/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ZMBV_fuzzer-5738330308739072

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>