fix crash in mapserver (introduced with 1215d0b)

Author: jef-n

src/providers/postgres/qgspostgresconn.cpp

@@ -270,7 +270,7 @@ void QgsPostgresConn::disconnect()
   Q_ASSERT( !key.isNull() );
   connections.remove( key );
 
-  if ( QThread::currentThread() == QApplication::instance()->thread() )
+  if ( !QApplication::instance() || QThread::currentThread() == QApplication::instance()->thread() )
     deleteLater();
   else
     delete this;