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>

libavcodec/hevc.c

@@ -283,7 +283,7 @@ static int decode_lt_rps(HEVCContext *s, LongTermRPS *rps, GetBitContext *gb)
 static int set_sps(HEVCContext *s, const HEVCSPS *sps)
 {
     int ret;
-    int num = 0, den = 0;
+    unsigned num = 0, den = 0;
 
     pic_arrays_free(s);
     ret = pic_arrays_init(s, sps);