hevc_mvs: initialize the temporal MV in case of missing ref

The caller expects the MV to always be initialized.

libavcodec/hevc_mvs.c

@@ -257,8 +257,10 @@ static int temporal_luma_motion_vector(HEVCContext *s, int x0, int y0,
 
     HEVCFrame *ref = s->ref->collocated_ref;
 
-    if (!ref)
+    if (!ref) {
+        memset(mvLXCol, 0, sizeof(*mvLXCol));
         return 0;
+    }
 
     tab_mvf = ref->tab_mvf;
     colPic  = ref->poc;