luci-lib-ip: fix segfault in link() on systems with ip6gre support

luci-lib-ip: fix segfault in link() on systems with ip6gre support

If an ip6gre interface is queried, the resulting link local address string
of 47 byte overflowed the buffer of 32 bytes used to hold the translated
address.

Increase the address string buffer to 48 byte and check the address length
prior to writing it into the buffer to avoid future problems with longs
link local addresses.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>