Limits based on 3GPP TS 26.245 V14.0.0
Fixes: Timeout
Fixes: 6377/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOVTEXT_fuzzer-5175929115508736

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>