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 6e5f8d48, 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

src/bin/psql/command.c

@@ -2974,6 +2974,11 @@ do_connect(enum trivalue reuse_previous_specification,
 			reuse_previous = !has_connection_string;
 			break;
 	}
+
+	/* If the old connection does not exist, there is nothing to reuse. */
+	if (!o_conn)
+		reuse_previous = false;
+
 	/* Silently ignore arguments subsequent to a connection string. */
 	if (has_connection_string)
 	{