Fix crash in pg_replication_slot_advance
We were trying to use a LSN variable after releasing its containing slot
structure.
Reported by: tushar
Author: amul sul
Reviewed-by: Petr Jelinek, Masahiko Sawada
Discussion: https://postgr.es/m/
94ba999c-f76a-0423-6523-
b8d531dfe4c7@enterprisedb.com