avcodec/brenderpix: Check input size before allocating image

avcodec/brenderpix: Check input size before allocating image

An incomplete image is not supported prior to this and will
not produce any output. This commit moves the failure before
time consuming operations.

Fixes: Timeout (81sec -> 76ms)
Fixes: 15723/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BRENDER_PIX_fuzzer-5147265653538816
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>