Valgrind: Guard against missing logger

Change-Id: I2fa2df124b83be12c2156f944d2ecaece9e51eac
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/valgrind/memcheck/memcheckrunner.cpp

@@ -168,6 +168,7 @@ void MemcheckRunner::logSocketConnected()
 
 void MemcheckRunner::readLogSocket()
 {
+    QTC_ASSERT(d->logSocket, return);
     emit logMessageReceived(d->logSocket->readAll());
 }