Check that we have a cursor to restore

Forgot to check that we actually have a cursor from the server
before trying to switch back to it after dismissing the context menu.
diff --git a/vncviewer/Viewport.cxx b/vncviewer/Viewport.cxx
index 4b8a85b..733e4a2 100644
--- a/vncviewer/Viewport.cxx
+++ b/vncviewer/Viewport.cxx
@@ -931,7 +931,7 @@
 
   // Back to our proper mouse pointer.
 #ifdef HAVE_FLTK_CURSOR
-  if (Fl::belowmouse() == this)
+  if ((Fl::belowmouse() == this) && cursor)
     window()->cursor(cursor, cursorHotspot.x, cursorHotspot.y);
 #endif