bug#1540: emacsclient -t fails under su

[Glenn Morris]

I'm logged into an X session as user1.
>From the xterm associated with /dev/pts/12, I do:

user1> su - user2
user2> emacs -Q -nw

That works fine. Then I exit that Emacs and do:

user2> emacs -Q -f server-start &    # works fine
user2> emacsclient -c                # works fine
user2> emacsclient -t
*ERROR*: Could not open file: /dev/pts/12

(and indeed user2 does not have permissions for this file)

user2> emacsclient -t

This second attempt kills Emacs with a segfault.