platform/linux: fix setting of IFA_ADDRESS without a peer

Since commit 9ecdba31 ('platform: create netlink messages directly
without libnl-route-3') we're unconditionally setting IFA_ADDRESS to
the peer address, even if there's no peer and it's all zeroes.

The kernel actually stopped caring somewhere around commit caeaba790
('ipv6: add support of peer address') in v3.10, but Ubuntu Touch likes
tu run Android's v3.4 with on some poorly supported hardware.

Fixes: 9ecdba31
7 jobs for lr/poor-florian in 15 minutes and 28 seconds
latest
Status Job ID Name Coverage
  Test
passed #97518
allowed to fail
checkpatch

00:02:00

passed #97519
f28_build_autotools

00:13:54

passed #97520
f28_build_meson

00:09:34

passed #97521
f28_rpm_autotools_dbg

00:12:52

passed #97522
f28_rpm_meson_dbg

00:11:47

passed #97523
f_latest_build_autotools

00:13:27

failed #97524
allowed to fail
f_rawhide_build_autotools

00:04:37

 
Name Stage Failure
failed
f_rawhide_build_autotools Test
  CC       src/devices/wwan/libnm_device_plugin_wwan_la-nm-device-modem.lo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:14562: src/devices/wwan/libnm_wwan_la-nm-modem-broadband.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make[1]: *** [Makefile:18310: all-recursive] Error 1
make[1]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make: *** [Makefile:8169: all] Error 2
ERROR: Job failed: exit code 1