atspicontext: Cache empty atspi bus address

We are caching the bus address as data on the display object when it
exists, but fail to set the data when the bus address doesn't exist.
That causing excessive calls to GetAddress when the accesssbility
bus doesn't exist. Make sure to cache a non-existent accessibility
bus by setting the "" string.

gtk/a11y/gtkatspicontext.c

@@ -1732,6 +1732,12 @@ get_bus_address (GdkDisplay *display)
 #endif
 
 out:
+
+  if (bus_address == NULL)
+    g_object_set_data_full (G_OBJECT (display), "-gtk-atspi-bus-address",
+                            g_strdup (""),
+                            g_free);
+
   return bus_address;
 }