avcodec/h264_refs: Assert that op is 0-2

Coverity fails to realize this
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_refs.c

@@ -368,6 +368,8 @@ int ff_h264_build_ref_list(H264Context *h, H264SliceContext *sl)
                 }
                 break;
             }
+            default:
+                av_assert1(0);
             }
 
             if (i < 0) {