Bug 24538: Handle Net::Netmask parser errors

Bug 24538: Handle Net::Netmask parser errors

This patch switches from the new() to new2() constructor,
which will return an undef value when it fails to parse a value.

This patch warns on parser failures, but otherwise silently drops
the invalid value, and returns objects for any valid input it can parse.
This way one mistake won't disable the whole feature.

To test:
1. Run the unit test t/Koha/Middlware/RealIP.t

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>