avcodec/dfa: Fix signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'

Fixes: 1368/clusterfuzz-testcase-minimized-4507293276176384

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