h264: reset ref count if decoding the slice header fails

Otherwise the ER code might try to use some already freed references.

Fixes possible access to freed memory.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/h264.c

@@ -4813,9 +4813,10 @@ again:
                 context_count = 0;
             }
 
-            if (err < 0)
+            if (err < 0) {
                 av_log(h->avctx, AV_LOG_ERROR, "decode_slice_header error\n");
-            else if (err == 1) {
+                h->ref_count[0] = h->ref_count[1] = h->list_count = 0;
+            } else if (err == 1) {
                 /* Slice could not be decoded in parallel mode, copy down
                  * NAL unit stuff to context 0 and restart. Note that
                  * rbsp_buffer is not transferred, but since we no longer