Windows : Fix crash with "uninitialized variable keyname used"

by C runtime debug check.

sql/sql_table.cc

@@ -5843,7 +5843,7 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
     Key *key;
     List_iterator<Key> key_it(alter_info->key_list);
     uint n_key;
-    const char *keyname;
+    const char *keyname= NULL;
     while ((key=key_it++))
     {
       if (!key->if_not_exists() && !key->or_replace())