BUG #16330: psql accesses null pointer in connect.c:do_connect

The following bug has been logged on the website:

Bug reference:      16330
Logged by:          Hugh Wang
Email address:      hghwng@gmail.com
PostgreSQL version: 12.2
Operating system:   Arch Linux
Description:        

If the connection to postmaster is closed, then trying to re-connect to
another one leads to SIGSEGV.

REPRODUCE:
$ psql
-> \conninfo
You are connected to database "hugh" as user "hugh" via socket in
"/run/postgresql" at port "5432".
*shut down server with commands like "systemctl stop postgresql"*
-> \conninfo 
You are currently not connected to a database.
-> \c a b c d
[1]    984978 segmentation fault (core dumped)  psql

ANALYSIS:
PQhost(o_conn) returns NULL, and strcmp(host, NULL) raises SIGSEGV.

SOURCE:
https://github.com/postgres/postgres/blob/master/src/bin/psql/command.c#L3016