avcodec/hevc_ps: Fix max_dec_buffer check

Fixes: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 2339/clusterfuzz-testcase-minimized-6663164320022528

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