ptsname test

On all platforms, ptsname() sets errno when the argument is an invalid
file descriptor. Even through POSIX does not require it [1], it's safe to
check it.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/ptsname.html

2012-06-24  Bruno Haible  <bruno <at> clisp.org>

	ptsname test: Extend test.
	* tests/test-ptsname.c: Include <errno.h>.
	(main): Test behaviour with invalid file descriptor.

--- tests/test-ptsname.c.orig	Sun Jun 24 14:51:29 2012
+++ tests/test-ptsname.c	Sun Jun 24 14:42:35 2012
 <at>  <at>  -21,6 +21,7  <at>  <at> 
 #include "signature.h"
 SIGNATURE_CHECK (ptsname, char *, (int));

+#include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
 <at>  <at>  -63,6 +64,17  <at>  <at> 
 #endif

   {
+    char *result;
+
+    errno = 0;
+    result = ptsname (-1);