MDEV-30680 Warning: Memory not freed: 280 on mangled query, LeakSanitizer: detected memory leaks

The parser works as follows:

The rule expr_lex returns a pointer to a newly created sp_expr_lex
instance which is not linked to any MariaDB structures yet - it is
pointed only from a Bison stack variable. The sp_expr_lex instance
gets linked to other structures (such as sp_instr_jump_if_not) later,
after scanning some following grammar.

Problem before the fix:
If a parse error happened immediately after expr_lex (before it got linked),
the created sp_expr_lex value got lost causing a memory leak.

Fix:

- Using Bison's "destructor" directive to free the results of expr_lex
  on parse/oom errors.

- Moving the call for LEX::cleanup_lex_after_parse_error() from
  MYSQL_YYABORT and yyerror inside parse_sql().
  This is needed because Bison calls destructors after yyerror(),
  while it's important to delete the sp_expr_lex instance before
  LEX::cleanup_lex_after_parse_error().
  The latter frees the memory root containing the sp_expr_lex instance.

  After this change the code block are executed in the following order:

  - yyerror() -- now only raises the error to DA (no cleanup done any more)
  - %destructor { delete $$; } <expr_lex>  -- destructs the sp_expr_lex instance
  - LEX::cleanup_lex_after_parse_error()   -- frees the memory root containing
                                              the sp_expr_lex instance

- Removing the "delete sublex" related code from restore_lex():
  - restore_lex() is called in most cases on success, when delete is not needed.
  - There is one place when restore_lex() is called on error:
    In sp_create_assignment_instr(). But in this case LEX::sp_lex_in_use
    is true anyway.
    The patch adds a new DBUG_ASSERT(lex->sp_lex_in_use) to guard this.

sql/sql_parse.cc

@@ -10454,6 +10454,17 @@ bool parse_sql(THD *thd, Parser_state *parser_state,
          ((thd->variables.sql_mode & MODE_ORACLE) ?
           ORAparse(thd) :
           MYSQLparse(thd)) != 0;
+
+  if (mysql_parse_status)
+  {
+    /*
+      Restore the original LEX if it was replaced when parsing
+      a stored procedure. We must ensure that a parsing error
+      does not leave any side effects in the THD.
+    */
+    LEX::cleanup_lex_after_parse_error(thd);
+  }
+
   DBUG_ASSERT(opt_bootstrap || mysql_parse_status ||
               thd->lex->select_stack_top == 0);
   thd->lex->current_select= thd->lex->first_select_lex();