avcodec/aacps: Fix undefined behavior

Fixes: 1337/clusterfuzz-testcase-minimized-5212314171080704

Fixes the existence of a potentially invalid pointer intermediate

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