Create IpPrefix in IaPrefixOption constructor

The constructor is invoked during Struct#parse() at which point we might
as well try to create the IpPrefix object which will be used later. If
this fails for some reason (UnknownHostException or ClassCastException),
the prefix is bogus and should not be used.

Test: TH
Change-Id: I7e0090735022d017ba11aa85266ba19e65807da7