avcodec/hevcdec: Fix signed integer overflow in decode_lt_rps()

Fixes: runtime error: signed integer overflow: 2147483647 + 6 cannot be represented in type 'int'
Fixes: 2263/clusterfuzz-testcase-minimized-4800359627227136

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