Assert that we have a socketEngine before calling it

If we are connected or bound, then we must have an engine.
Otherwise in code we frequently test for the d->socketEngine pointer,
including in waitForConnected.

Fixes static anlyzer warning 3b37ce4102843c97ceef2b7bb68a409d.

Pick-to: 6.1
Change-Id: Ib9818282ad2813db97859cbe1f184bcfb5ea4ab4
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/socket/qabstractsocket.cpp

@@ -2197,6 +2197,7 @@ bool QAbstractSocket::waitForReadyRead(int msecs)
     do {
         if (state() != ConnectedState && state() != BoundState)
             return false;
+        Q_ASSERT(d->socketEngine);
 
         bool readyToRead = false;
         bool readyToWrite = false;