Prevent Internal Server Error if range is backwards

Fixes redmine #3950 - ip_range_to_subnet_array can easily swap the input parameters if the caller has passed/entered them the wrong way around. That is both friendly to the caller and ensures that a hostile caller can't blow up the routine.
This patches 2.1 branch - will submit pull request for master also.
Merge pull request #1312 from phil-davis/patch-8

Prevent Internal Server Error if range is backwards