iptables-xml: fix segfault if missing space after -A

As pointed out by Bernhard Reutner-Fischer, a malformed line fed to
iptables-xml such as the below with a missing space after the -A:

	-APOSTROUTING -d 1.1.1.1/32 -p tcp -j MASQUERADE

causes a segfault.  Patch attached.

This closes netfilter bugzilla #886.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>