Fixes: out of array access
Fixes: 62467/clusterfuzz-testcase-minimized-ffmpeg_BSF_EVC_FRAME_MERGE_fuzzer-6092990982258688

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: "Dawid Kozinski/Multimedia (PLT) /SRPOL/Staff Engineer/Samsung Electronics" <d.kozinski@samsung.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>