Fix crash in psql when attempting to reuse old connection

 Fix crash in psql when attempting to reuse old connection
 
 In a psql session, if the connection to the server is abruptly cut, the
 referenced connection would become NULL as of CheckConnection().  This
 could cause a hard crash with psql if attempting to connect by reusing
 the past connection's data because of a null-pointer dereference with
 either PQhost() or PQdb().  This issue is fixed by making sure that no
 reuse of the past connection is done if it does not exist.
 
 Issue has been introduced by 6e5f8d4, so backpatch down to 12.
 
 Reported-by: Hugh Wang
 Author: Michael Paquier
 Reviewed-by: Álvaro Herrera, Tom Lane
 Discussion: https://postgr.es/m/16330-b34835d83619e25d@postgresql.org
 Backpatch-through: 12