sifpidx (an interface name) cannot extend beyond the end of the
fr_names buffer.
We do the validation for fr_sifpidx here because it is a union that
contains an offset only when fr_sifpidx points to an interface name,
an offset into fr_names. The union is an offset into fr_names in this
case only.

Note that sifpidx is only used in ipf_sync() which implments ipf -y.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 1 week