QSSLSocket::readData return -1 when socket is not connected

As QAbstractSocket::readData does and as the documentation of QIODevice says
"this function returns -1 in those cases (that is, reading on a closed
socket..."

Change-Id: I1e64673f6a6d792a640bd6cb28b2bb5a0f18dc36
Reviewed-by: Aleix Pol
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/network/ssl/qsslsocket.cpp

@@ -1993,6 +1993,8 @@ qint64 QSslSocket::readData(char *data, qint64 maxlen)
         // possibly trigger another transmit() to decrypt more data from the socket
         if (d->plainSocket->bytesAvailable())
             QMetaObject::invokeMethod(this, "_q_flushReadBuffer", Qt::QueuedConnection);
+        else if (d->state != QAbstractSocket::ConnectedState)
+            return maxlen ? qint64(-1) : qint64(0);
     }
 
     return readBytes;

tests/auto/network/ssl/qsslsocket/tst_qsslsocket.cpp

@@ -2162,6 +2162,13 @@ void tst_QSslSocket::waitForMinusOne()
 
     // fifth verification: it should wait for 200 ms more
     QVERIFY(socket.waitForDisconnected(-1));
+
+    // sixth verification: reading from a disconnected socket returns -1
+    //                     once we deplete the read buffer
+    QCOMPARE(socket.state(), QAbstractSocket::UnconnectedState);
+    socket.readAll();
+    char aux;
+    QCOMPARE(socket.read(&aux, 1), -1);
 }
 
 class VerifyServer : public QTcpServer