Make vncRandRGetOutputDimensions tolerant for disabled outputs

Return error if no CRTC.
diff --git a/unix/common/randr.cxx b/unix/common/randr.cxx
index 8581991..b0adc3f 100644
--- a/unix/common/randr.cxx
+++ b/unix/common/randr.cxx
@@ -65,9 +65,9 @@
       newIdMap[outputId] = id;
     }
 
-    vncRandRGetOutputDimensions(i, &x, &y, &width, &height);
-
-    layout.add_screen(rfb::Screen(newIdMap[outputId], x, y, width, height, 0));
+    if (vncRandRGetOutputDimensions(i, &x, &y, &width, &height) == 0) {
+      layout.add_screen(rfb::Screen(newIdMap[outputId], x, y, width, height, 0));
+    }
   }
 
   /* Only keep the entries that are currently active */