avcodec/h264: Only reinit quant tables if a new PPS is allowed

Fixes null pointer dereference
Fixes: signal_sigsegv_3042097_3007_cov_1741463594_non_monotone_timestamps1.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_slice.c

@@ -1497,7 +1497,7 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
         }
     }
 
-    if (h == h0 && h->dequant_coeff_pps != pps_id) {
+    if (first_slice && h->dequant_coeff_pps != pps_id) {
         h->dequant_coeff_pps = pps_id;
         h264_init_dequant_tables(h);
     }