Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

src/ui/frames.c

@@ -955,6 +955,7 @@ grab_op_from_resize_control (MetaFrameControl control)
       return META_GRAB_OP_RESIZING_W;
     default:
       g_assert_not_reached ();
+      return META_GRAB_OP_NONE;
     }
 }
 
@@ -1003,6 +1004,7 @@ get_button_number (const ClutterEvent *event)
     return clutter_event_get_button (event);
 
   g_assert_not_reached ();
+  return -1;
 }
 
 static gboolean
@@ -1105,6 +1107,7 @@ meta_frame_left_click_event (MetaUIFrame        *frame,
       return FALSE;
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }