h264: set ref_count to 0 for intra slices.

CC:libav-stable@libav.org
(cherry picked from commit 437211ae73)

Fixes deadlocks waiting for non-existing references with some fuzzed files.

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/h264.c

@@ -3039,8 +3039,11 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             h->list_count = 2;
         else
             h->list_count = 1;
-    } else
+    } else {
         h->list_count = 0;
+        h->ref_count[0] = h->ref_count[1] = 0;
+    }
+
 
     max_refs = s->picture_structure == PICT_FRAME ? 16 : 32;