h264: check ref_count validity for num_ref_idx_active_override_flag

Fixes segfault in the fuzzed sample bipbop234.ts_s226407.

CC: libav-stable@libav.org

libavcodec/h264.c

@@ -2841,8 +2841,13 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
 
         if (num_ref_idx_active_override_flag) {
             h->ref_count[0] = get_ue_golomb(&s->gb) + 1;
-            if (h->slice_type_nos == AV_PICTURE_TYPE_B)
+            if (h->ref_count[0] < 1)
+                return AVERROR_INVALIDDATA;
+            if (h->slice_type_nos == AV_PICTURE_TYPE_B) {
                 h->ref_count[1] = get_ue_golomb(&s->gb) + 1;
+                if (h->ref_count[1] < 1)
+                    return AVERROR_INVALIDDATA;
+            }
         }
 
         if (h->slice_type_nos == AV_PICTURE_TYPE_B)