xgettext: Fix double-free in singular/plural argument extraction
After commit 6aa7b7ed in 2009, xgettext assumed that ARGNUM1 and
ARGNUM2 of -k are different. That could cause an double-free in
exceptional cases.
Reported by Johan Liljegren in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-10/msg00028.html>.
* xgettext.c (arglist_parser_remember_literal): Don't assume that
ARGNUM1 and ARGNUM2 are different.
