paned: Initialize clip properly

This was causing problems in the case when only one of the paned
children is visible - we would use uninitialized memory, leading
to invalide clip regions. Concretely, the signal tab in the inspector
would sometimes not render at all.

gtk/gtkpaned.c

@@ -1357,7 +1357,7 @@ gtk_paned_allocate (GtkCssGadget        *gadget,
   GtkWidget *widget = gtk_css_gadget_get_owner (gadget);
   GtkPaned *paned = GTK_PANED (widget);
   GtkPanedPrivate *priv = paned->priv;
-  GtkAllocation clip;
+  GtkAllocation clip = { 0 };
 
   if (priv->child1 && gtk_widget_get_visible (priv->child1) &&
       priv->child2 && gtk_widget_get_visible (priv->child2))
@@ -1477,7 +1477,6 @@ gtk_paned_allocate (GtkCssGadget        *gadget,
             child2_allocation.height = child2_height;
         }
 
-
       gtk_css_gadget_allocate (priv->handle_gadget,
                                &priv->handle_pos,
                                -1,