pg_stat_statements: remove redundant function call in pg_stat_statements_internal

Hi,

I found some redundant function calls in
pg_stat_statements.c/pg_stat_statements_internal(),
There is no need to call GetUserId() again because the value was
previously obtained.
so I propose a patch to fix it.

--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -1508,7 +1508,7 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
        pgssEntry  *entry;

        /* Superusers or members of pg_read_all_stats members are allowed */
-       is_allowed_role = is_member_of_role(GetUserId(),
ROLE_PG_READ_ALL_STATS);
+       is_allowed_role = is_member_of_role(userid, ROLE_PG_READ_ALL_STATS);

        /* hash table must exist already */
        if (!pgss || !pgss_hash)

Regards,
Lee Dong Wook.