Check whether the iface exists before removing

network in teardown.

If the setup function exists early due to missing
P2P support, the iface will not exist when we
get to the teardown step. This will lead to a
null pointer exception.

Bug: 387556129
Test: Rerun failing test on Treehugger
Change-Id: I3bdc956afd60eea10453b864f498259729abc182