libifconfig: Fix bridge status member list

When this functionality was moved to libifconfig in 3dfbda3401abea84da9,
the end of list calculation was modified for unknown reasons, practically
limiting the number of bridge member returned to (about) 102.

This patch changes the calculation back to what it was originally and
adds a unit test to verify it works as expected.

Reported by: Patrick M. Hausen (via ML)
Reviewed by: kp
Approved by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43135