There is no outright bug here but it is better to be consistent with the
usage at other places in the same file. In the passing, fix a wrong
assertion in pgstat_recv_replslot.

Author: Kyotaro Horiguchi
Reviewed-by: Sawada Masahiko and Amit Kapila
Discussion: https://postgr.es/m/20201104.175523.1704166915688949637.horikyota.ntt@gmail.com