shapewipe: Ensure caps are writable

The exist one case where that we endup with original caps in ret, in which
case we are not guaratied to have writable caps. Simply ensure this is the
caps are writable before entering the loop.

https://bugzilla.gnome.org/show_bug.cgi?id=700044

gst/shapewipe/gstshapewipe.c

@@ -486,6 +486,7 @@ gst_shape_wipe_mask_sink_getcaps (GstShapeWipe * self, GstPad * pad,
   if (gst_caps_is_empty (ret))
     goto done;
 
+  ret = gst_caps_make_writable (ret);
   n = gst_caps_get_size (ret);
   tmp = gst_caps_new_empty ();
   for (i = 0; i < n; i++) {