videorate: GstStructure refcount critical message

s3 is not being initialized when run in a loop
and the same was being freed, which resulted in the crash

https://bugzilla.gnome.org/show_bug.cgi?id=735952

gst/videorate/gstvideorate.c

@@ -366,6 +366,7 @@ gst_video_rate_transform_caps (GstBaseTransform * trans,
 
     s1 = gst_structure_copy (s);
     s2 = gst_structure_copy (s);
+    s3 = NULL;
 
     if (videorate->drop_only) {
       gint min_num = 0, min_denom = 1;