ip route: get: allow zero-length subnet mask

ip route: get: allow zero-length subnet mask

A /0 subnet mask is theoretically valid, but ip route get doesn't allow
it:

$ ip route get 1.0.0.0/0
need at least a destination address

Change the check and remember whether we found an address or not, since
according to the documentation it's a mandatory parameter.

$ ip/ip route get 1.0.0.0/0
1.0.0.0 via 192.168.1.1 dev eth0 src 192.168.1.91 uid 1000
    cache

Reported-by: Clément Hertling <wxcafe@wxcafe.net>
Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>