Bug#889720: xauth crashes when directory name matches host name

[John Williams <john-debian@pond-weed.com>]

Package: xauth
Version: 1:1.0.10-1
Architecture: arm64

Running 'startx' on a machine named 'myhost' (aarch64), with a sub-directory named 'myhost' in my home directory:  X starts very slowly, and I see the message:

	xauth:  timeout in locking authority file /home/john/.Xauthority

Here's what happens: the 'startx' script calls 'xauth list' in a loop (line 199) and the first call crashes after receiving SIG_SEGV; this leaves its lock files in place, and the next instance of xauth times out waiting for them to go away.

I think I've traced the reason for 'xauth list' crashing: my .Xauthority file contains a line like this:

	myhost:0 MIT-MAGIC-COOKIE-1 <some value>

and while processing this for printing, there's some code in parsedpy.c (line 178) that does this:

	if (0 == stat(path, &sbuf))
		family = FamilyLocal;

and this test succeeds because 'myhost' is the name of a valid file (actually a directory)

Then later on in gethost.c we have:

	switch (family) {
		case FamilyLocal: /* hostname/unix:0 */
			...
			if (0 == stat(path, &sbuf))
				is_path_to_socket = 1;
			...
			if (is_path_to_socket)
				strncpy(buf, strrchr(fulldpyname, '/') + 1, sizeof(buf));

But there is no '/' in the name, so we end up passing a null pointer to strncpy(), which crashes the whole program.