[aurorae] Use QMutexLocker in AuroraeClient's dtor

It's possible that the rendering thread is still writing to the
buffer and if we destroy the buffer before it's finished KWin is going
to crash. So let's mutex lock the dtor to ensure that the rendering
thread finishes before we tear down the client.

BUG: 336950

clients/aurorae/src/aurorae.cpp

@@ -296,6 +296,7 @@ AuroraeClient::AuroraeClient(KDecorationBridge *bridge, KDecorationFactory *fact
 
 AuroraeClient::~AuroraeClient()
 {
+    QMutexLocker locker(AuroraeFactory::instance()->mutex());
 }
 
 void AuroraeClient::init()