avcodec/hevc: Dont turn 32bit timebases into negative numbers

Fixes assertion failure
Fixes: 499f6ecaeae8e3f79a115a72c61ffd15-asan_static-oob_124a175_2792_cov_1782273879_RPLM_A_qualcomm_4.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
