Fix 'Free swap space in %' calculation so that it does not divide by zero

[hlovdal]

The original calculation

last(//system.swap.free[memAvailSwap.0]) / last(//system.swap.total[memTotalSwap.0]) * 100

will fail with a divide by zero error when system.swap.total[memTotalSwap.0] equals zero. To avoid that I changed the calculation to

(last(//system.swap.total[memTotalSwap.0]) <> 0) * last(//system.swap.free[memAvailSwap.0]) /
(last(//system.swap.total[memTotalSwap.0]) + (last(//system.swap.total[memTotalSwap.0]) = 0) ) * 100

So when memTotalSwap.0 equals zero then the expression evaluates to zero:

==> (0) * last(//system.swap.free[memAvailSwap.0]) / (last(//system.swap.total[memTotalSwap.0]) + (1) ) * 100
==> 0 * last(//system.swap.free[memAvailSwap.0]) / 1 * 100
==> 0

and when memTotalSwap.0 is different from zero the expression evaluates to the original:

==> (1) * last(//system.swap.free[memAvailSwap.0]) / (last(//system.swap.total[memTotalSwap.0]) + (0) ) * 100
==> last(//system.swap.free[memAvailSwap.0]) / last(//system.swap.total[memTotalSwap.0]) * 100