Fixes: signed integer overflow: -9223372036854767583 + -65536 cannot be represented in type 'long'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_FLV_fuzzer-6734549467922432
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>