Alternatively these conditions could be treated as errors
Fixes: 23147/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5639254549200896
Fixes: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'int64_t' (aka 'long')

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 584d334afd59714ed04637a9227a4f1368c26166)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>