Only focusToNull when activating a ShellClient

ShellClient::doSetActive calls into focusToNull to deactivate active
windows on X11. But this should only be done when activating a
ShellClient. When deactivating a ShellClient the focusToNull can
result in an activated X-client to lose activation.

Thus this change ensures that focusToNull is only performed for a
now active ShellClient.

Reviewed-By: bshah

shell_client.cpp

@@ -789,6 +789,9 @@ void ShellClient::takeFocus()
 
 void ShellClient::doSetActive()
 {
+    if (!isActive()) {
+        return;
+    }
     StackingUpdatesBlocker blocker(workspace());
     workspace()->focusToNull();
 }