Fix a segfault when the wayland connection is broken

When calling ::exit() the socket notifier in the events thread may still
fire before the process actually exits, using objects that are being
destroyed and resulting in a segfault.
Stop the events thread before calling ::exit().

Change-Id: I187762da2a7efa83db1e62b0e28dfab89f478c7d
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>

src/plugins/platforms/wayland/qwaylanddisplay.cpp

@@ -194,6 +194,8 @@ void QWaylandDisplay::blockingReadEvents()
 
 void QWaylandDisplay::exitWithError()
 {
+    mEventThread->quit();
+    mEventThread->wait();
     ::exit(1);
 }