check_swap: returns OK, if no swap activated [sf#2823005]

My swap was not activated on boot for unknown reason and nagios does not
report this as a problem. Here is an example:

[root@kecom ~]# rpm -q nagios-plugins
nagios-plugins-1.4.13-11.fc10.i386
[root@kecom ~]# /usr/lib/nagios/plugins/check_swap -w 80% -c 40% -c 1 -w 2
SWAP CRITICAL - 100% free (0 MB out of 0 MB) |swap=0MB;0;0;0;0

If there is no swap and users is trying to test percentage of free swap,
consider 0 MB free swap space as problem, or of free/total raises division
by zero, then set percentage to 0%, not to 100%.
--
Just turning attached patch of github issue #896 into a push request.
(Closes #896)