avcodec/dpx: Check elements in 12bps planar path
Fixes: null pointer dereference
Fixes: 8946/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DPX_fuzzer-
5078915222601728
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit
75a2db552423295b509546f3b0f8b2b46d3424b1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>