The headers from where the dimensions are read in actual files
are limited to 16bit per component.

Fixes: Timeout
Fixes: 6305/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DFA_fuzzer-4824270749302784

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>