ptx: avoid heap overrun for backslash at end of optarg string

ptx: avoid heap overrun for backslash at end of optarg string

* src/ptx.c (copy_unescaped_string): Ignore a lone backslash
at end of string.  Reported by Cristian Cadar, Daniel Dunbar
and Dawson Engler.  Details here:
<http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/13005>.
* tests/misc/Makefile.am (TESTS): Add ptx-overrun.
* tests/misc/ptx-overrun: New file.  Test for the above fix.
* NEWS: Mention the fix.

Signed-off-by: Jim Meyering <meyering@redhat.com>