ClutterX11TexturePixmap: Fix unbalanced error trapping

I think this is what commit 2cf1405506 intended to do since it
specifically mentioned cleaning up the trap in
clutter_x11_texture_pixmap_set_pixmap, but although it moved the untrap
to only be done in the case where Pixmap != None it left the position of
the trap itself unchanged. This meant the error trapping wouldn't be
balanced if pixmap == None since the untrap wouldn't be done. We now
only trap and untrap around the XGetGeometry call done when pixmap !=
None.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2303

clutter/x11/clutter-x11-texture-pixmap.c

@@ -797,8 +797,6 @@ clutter_x11_texture_pixmap_set_pixmap (ClutterX11TexturePixmap *texture,
 
   priv = texture->priv;
 
-  clutter_x11_trap_x_errors ();
-
   /* Get rid of the existing Cogl texture early because it may try to
      use the pixmap which we might destroy */
   material = clutter_texture_get_cogl_material (CLUTTER_TEXTURE (texture));
@@ -807,6 +805,7 @@ clutter_x11_texture_pixmap_set_pixmap (ClutterX11TexturePixmap *texture,
 
   if (pixmap != None)
     {
+      clutter_x11_trap_x_errors ();
       status = XGetGeometry (clutter_x11_get_default_display(),
 			     (Drawable)pixmap,
 			     &root,