Sanity check when cleaning up keymap changes

Make sure we don't send a bogus request to the X server in the (common)
case that we don't actually have anything to restore.

unix/x0vncserver/XDesktop.cxx

     }
   }
 
+  // Did we actually find something to remove?
+  if (highestKeyCode < lowestKeyCode)
+    return;
+
   changes.changed |= XkbKeySymsMask;
   changes.first_key_sym = lowestKeyCode;
   changes.num_key_syms = highestKeyCode - lowestKeyCode + 1;