avcodec/h264_refs: Change default case to av_assert0() to suppress warning

Should fix "libavcodec/h264_refs.c:372:13: warning: variable 'i' is used uninitialized whenever switch default is taken"

Found-by: durandal_17
Suggested-by: jkqxz
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_refs.c

@@ -370,7 +370,7 @@ int ff_h264_build_ref_list(H264Context *h, H264SliceContext *sl)
                 break;
             }
             default:
-                av_assert1(0);
+                av_assert0(0);
             }
 
             if (i < 0) {