Fix valgrind complaints on alter_default_table_charset

Summary:
As title, fixed valgrind complaints on alter_default_table_charset in the code
path of `create database` (blame D56013)

squash with 8194409 Per Database Read-Only
(no need for the summary)

Test Plan: Manually ran a failed valgrind test and passed. Also see Jenkins.

Reviewers: santoshb

Reviewed By: santoshb

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D56061

Author: tianx

sql/sql_yacc.yy

@@ -2738,6 +2738,7 @@ create:
         | CREATE DATABASE opt_if_not_exists ident
           {
             Lex->create_info.default_table_charset= NULL;
+            Lex->create_info.alter_default_table_charset= false;
             Lex->create_info.used_fields= 0;
             Lex->create_info.db_read_only=
               enum_db_read_only::DB_READ_ONLY_NULL;