Fixes: signed integer overflow: -9223372036854775807 - 48000 cannot be represented in type 'long long'
Fixes: 26521/clusterfuzz-testcase-minimized-ffmpeg_dem_DIRAC_fuzzer-5635536506847232

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>