logical-monitor: Don't set the CRTC logical monitor if not assigned

When adding a monitor and all its outputs, don't try to set the logical
monitor of the outputs CRTC if none was assigned. This might happen if
a tiled monitor only uses a subset of the connectors it are connected
via.

https://bugzilla.gnome.org/show_bug.cgi?id=779745

src/backends/meta-logical-monitor.c

@@ -95,6 +95,7 @@ meta_logical_monitor_add_monitor (MetaLogicalMonitor *logical_monitor,
           MetaOutput *output = l_output->data;
 
           is_presentation = is_presentation && output->is_presentation;
+          if (output->crtc)
             output->crtc->logical_monitor = logical_monitor;
         }
     }