ip: fix unformat_ip_address forcing version to IP4 for some IP6 addresses

ip: fix unformat_ip_address forcing version to IP4 for some IP6 addresses

dd2f12ba made use of ip46_address_is_ip4() in order to determine whether
the address is ipv4 or ipv6 within unformat_ip_address - however, its
logic is correct only for some addresses. e.g. a valid IPv6 address of :: (unspecified)
will result in "true" result. This is probably not an issue for most
of the cases (the unspecified address is quite rare),
however if the unformat_ip_address is used as part of the
prefix parsing, the ::/0 is a fairly often utilized construct,
which gets parsed as 0.0.0.0

Solution: return the old logic, but use a temporary
variable to avoid overwriting the target memory on failure.

Type: fix
Fixes: dd2f12ba6ab952d9d66f4d9ba89ffde6309b1ff2.
Change-Id: I272f740dfdf07036cec68516e153f0701a53233d
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>