avcodec/h264: reset the counts in the correct context

Fixes null pointer dereference

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8f8d632220)

libavcodec/h264.c

@@ -1728,7 +1728,7 @@ again:
             if (err < 0 || err == SLICE_SKIPED) {
                 if (err < 0)
                     av_log(h->avctx, AV_LOG_ERROR, "decode_slice_header error\n");
-                h->ref_count[0] = h->ref_count[1] = h->list_count = 0;
+                hx->ref_count[0] = hx->ref_count[1] = hx->list_count = 0;
             } else if (err == SLICE_SINGLETHREAD) {
                 if (context_count > 1) {
                     ret = ff_h264_execute_decode_slices(h, context_count - 1);