monitor-config-manager: Handle comparing different sized config keys

The guard for handling size differences between keys were broken, it
only checked if the key passed by the second argument ended up being
shorter.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-config-manager.c

@@ -827,7 +827,7 @@ meta_monitors_config_key_equal (gconstpointer data_a,
         return FALSE;
     }
 
-  if (l_b || l_b)
+  if (l_a || l_b)
     return FALSE;
 
   return TRUE;