basetransform: don't propose_allocation before negotiation

Answer the allocation query with FALSE when we are not negotiated yet because at
that point we have no idea if we need to proxy the allocation query or not.

libs/gst/base/gstbasetransform.c

@@ -1362,13 +1362,20 @@ gst_base_transform_default_query (GstBaseTransform * trans,
   switch (GST_QUERY_TYPE (query)) {
     case GST_QUERY_ALLOCATION:
     {
-      GstQuery *decide_query;
+      GstQuery *decide_query = NULL;
+      gboolean negotiated;
 
       /* can only be done on the sinkpad */
       if (direction != GST_PAD_SINK)
         goto done;
 
       GST_OBJECT_LOCK (trans);
+      if (G_UNLIKELY (!(negotiated = trans->negotiated))) {
+        GST_DEBUG_OBJECT (trans,
+            "not negotiated yet, can't answer ALLOCATION query");
+        GST_OBJECT_UNLOCK (trans);
+        goto done;
+      }
       if ((decide_query = trans->priv->query))
         gst_query_ref (decide_query);
       GST_OBJECT_UNLOCK (trans);