xargs: restore correct behaviour of -n option

Since commit 1ff7002b1 (xargs: fix handling of quoted arguments, closes
11441) the -n option hasn't worked properly:

   $ echo 1 2 3 | xargs -n 1 echo
   1
   2

   3

   $

Because state is now remembered between calls to process_stdin() it's
necessary to update the state before any premature return.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>