avcodec/aacps: Fix multiple integer overflow in map_val_34_to_20()

Fixes: avcodec/aacps.c:511:40: runtime error: signed integer overflow: 1509077651 + 758068176 cannot be represented in type 'int'
Fixes: 2678/clusterfuzz-testcase-minimized-4702787684270080

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