avformat/avidec: Use av_sat_sub64() in check_stream_max_drift()

avformat/avidec: Use av_sat_sub64() in check_stream_max_drift()

Fixes: signed integer overflow: 8833900919969684211 - -9223372036854775808 cannot be represented in type 'long'
Fixes: 26726/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-5669377724383232
Fixes: 27587/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-6294562263531520
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>