avcodec/ffv1dec: Fix integer overflow in read_quant_table()

Fixes: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
Fixes: 3361/clusterfuzz-testcase-minimized-5065842955911168

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