avcodec/hevc_ps: fix integer overflow in log2_parallel_merge_level_minus2

Fixes: runtime error: signed integer overflow: -2147483647 - 2 cannot be represented in type 'int'
Fixes: 2702/clusterfuzz-testcase-minimized-4511932591636480

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