Bug #29419820: MEMORY LEAK IN MY_YYOVERFLOW()

Note: this patch is for 5.6.

Detected by ASAN.

The patch fixes the cleanup of parser stack pointers.
Reviewed-by: Guilhem Bichot <guilhem.bichot@oracle.com>

sql/sql_lex.h

-/* Copyright (c) 2000, 2015, Oracle and/or its affiliates.
-   Copyright (c) 2010, 2018, MariaDB Corporation
+/* Copyright (c) 2000, 2019, Oracle and/or its affiliates.
+   Copyright (c) 2010, 2019, MariaDB Corporation
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -2858,15 +2858,18 @@ class Set_signal_information
 class Yacc_state
 {
 public:
-  Yacc_state()
-  {
-    reset();
-  }
+  Yacc_state() : yacc_yyss(NULL), yacc_yyvs(NULL) { reset(); }
 
   void reset()
   {
-    yacc_yyss= NULL;
-    yacc_yyvs= NULL;
+    if (yacc_yyss != NULL) {
+      my_free(yacc_yyss);
+      yacc_yyss = NULL;
+    }
+    if (yacc_yyvs != NULL) {
+      my_free(yacc_yyvs);
+      yacc_yyvs = NULL;
+    }
     m_set_signal_info.clear();
     m_lock_type= TL_READ_DEFAULT;
     m_mdl_type= MDL_SHARED_READ;