h264: on reference overflow, reset the reference count to 0, not 1.

Since decode_slice_header() returns before the reference lists are
constructed, there are zero valid references.

CC:libav-stable@libav.org
(cherry picked from commit 668e16a0dd)

Conflicts:

	libavcodec/h264.c

libavcodec/h264.c

@@ -3046,7 +3046,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
 
     if (h->ref_count[0] > max_refs || h->ref_count[1] > max_refs) {
         av_log(h->s.avctx, AV_LOG_ERROR, "reference overflow\n");
-        h->ref_count[0] = h->ref_count[1] = 1;
+        h->ref_count[0] = h->ref_count[1] = 0;
         return AVERROR_INVALIDDATA;
     }