ghostpad: ensure we build a ghost pad ..

When we construct from a custom GType
diff --git a/gst/gstghostpad.c b/gst/gstghostpad.c
index dd53122..90cae38 100644
--- a/gst/gstghostpad.c
+++ b/gst/gstghostpad.c
@@ -635,6 +635,8 @@
         GST_PAD_TEMPLATE_GTYPE (templ) ==
         G_TYPE_NONE ? GST_TYPE_GHOST_PAD : GST_PAD_TEMPLATE_GTYPE (templ);
 
+    g_return_val_if_fail (g_type_is_a (pad_type, GST_TYPE_GHOST_PAD), NULL);
+
     ret = g_object_new (pad_type, "name", name,
         "direction", dir, "template", templ, NULL);
   } else {