Before freeing stmt in destructor check if dbh driver data is NULL (#829

)

* Issue 434 - set dbh driver data to NULL as well in destructor

* Reverted the last change but instead check if dbh driver_data is already freed

* Modified the comment

source/pdo_sqlsrv/pdo_stmt.cpp

@@ -496,8 +496,13 @@ int pdo_sqlsrv_stmt_dtor( _Inout_ pdo_stmt_t *stmt TSRMLS_DC )
     LOG( SEV_NOTICE, "pdo_sqlsrv_stmt_dtor: entering" );
 
     // if a PDO statement didn't complete preparation, its driver_data can be NULL
-    if( driver_stmt == NULL ) {
+    if (driver_stmt == NULL) {
+        return 1;
+    }
 
+    // occasionally stmt->dbh->driver_data is already freed and reset but its driver_data is not
+    if (stmt->dbh != NULL && stmt->dbh->driver_data == NULL) {
+        stmt->driver_data = NULL;
         return 1;
     }