avcodec/h264_mc_template: prefetch list1 only if it is used in the MB

Fixes ubsan warning
Fixes Mozilla bug 1230276

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_mc_template.c

@@ -158,6 +158,7 @@ static void MCFUNC(hl_motion)(const H264Context *h, H264SliceContext *sl,
         }
     }
 
+    if (USES_LIST(mb_type, 1))
         prefetch_motion(h, sl, 1, PIXEL_SHIFT, CHROMA_IDC);
 }