Prevent deleting non-existing support properties on the root window

Summary: Less likely to happen but still we need to handle this case.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23611

composite.cpp

@@ -508,6 +508,7 @@ void Compositor::deleteUnusedSupportProperties()
             // remove property from root window
             xcb_delete_property(con, kwinApp()->x11RootWindow(), atom);
         }
+        m_unusedSupportProperties.clear();
     }
 }