avcodec/diracdec: Change frame_number to 64bit as its a 32bit from the bitstream and we also have a -1 special case

Fixes: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
Fixes: 9291/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-6324345860259840

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