Fix an error propagation bug in tests/tun_interface.cpp

ifc_add_address() returns -errno on error, and doesn't necessarily
set 'errno' (for example if error came in via netlink message).

Test: builds
Bug: 142764715
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I9151649785d7c3cc4316d0e35e9b83dfa0d5053e