QPSQL: Fix crash when a prepared statement is deleted after the db was removed

When a prepared statement is still alive after the database was removed
with QSqlDatabase::removeDatabase(), the cleanup routine is trying to
access the driver which is no longer alive which results in a crash.
Fix it by checking if the driver is still alive similar to
5f66486c

.

Fixes: QTBUG-43889
Change-Id: Ib466a76d014e32c055d203bda15b075ad3dff3d9
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Jesus Fernandez <jsfdez@gmail.com>

src/plugins/sqldrivers/psql/qsql_psql.cpp

@@ -428,12 +428,14 @@ static QVariant::Type qDecodePSQLType(int t)
 
 void QPSQLResultPrivate::deallocatePreparedStmt()
 {
-    const QString stmt = QStringLiteral("DEALLOCATE ") + preparedStmtId;
-    PGresult *result = drv_d_func()->exec(stmt);
+    if (drv_d_func()) {
+        const QString stmt = QStringLiteral("DEALLOCATE ") + preparedStmtId;
+        PGresult *result = drv_d_func()->exec(stmt);
 
-    if (PQresultStatus(result) != PGRES_COMMAND_OK)
-        qWarning("Unable to free statement: %s", PQerrorMessage(drv_d_func()->connection));
-    PQclear(result);
+        if (PQresultStatus(result) != PGRES_COMMAND_OK)
+            qWarning("Unable to free statement: %s", PQerrorMessage(drv_d_func()->connection));
+        PQclear(result);
+    }
     preparedStmtId.clear();
 }