avcodec/h264_refs: Fix pps_ref_count with multiple PPS

Found-by: ubitux
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_refs.c

@@ -806,8 +806,10 @@ int ff_h264_execute_ref_pic_marking(H264Context *h, MMCO *mmco, int mmco_count)
     print_long_term(h);
 
     for (i = 0; i < FF_ARRAY_ELEMS(h->pps_buffers); i++) {
-        pps_ref_count[0] = FFMAX(pps_ref_count[0], h->pps.ref_count[0]);
-        pps_ref_count[1] = FFMAX(pps_ref_count[1], h->pps.ref_count[1]);
+        if (h->pps_buffers[i]) {
+            pps_ref_count[0] = FFMAX(pps_ref_count[0], h->pps_buffers[i]->ref_count[0]);
+            pps_ref_count[1] = FFMAX(pps_ref_count[1], h->pps_buffers[i]->ref_count[1]);
+        }
     }
 
     if (   err >= 0