platform/linux: fix setting of IFA_ADDRESS without a peer

Since commit 9ecdba316 ('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
to run Android's v3.4 on some poorly supported hardware.

Fixes: 9ecdba316c

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/77

src/platform/nm-linux-platform.c

@@ -7373,7 +7373,7 @@ ip6_address_add (NMPlatform *platform,
 	                             ifindex,
 	                             &addr,
 	                             plen,
-	                             &peer_addr,
+	                             IN6_IS_ADDR_UNSPECIFIED (&peer_addr) ? NULL : &peer_addr,
 	                             flags,
 	                             RT_SCOPE_UNIVERSE,
 	                             lifetime,