avcodec/aacps: Check border_position to be monotone

Fixes: runtime error: left shift of negative value -67108864
Fixes: 1738/clusterfuzz-testcase-minimized-6734814327603200

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