netdev: Verify ifa_addr is not NULL when iterating over getifaddrs.

netdev: Verify ifa_addr is not NULL when iterating over getifaddrs.

Some point-to-point devices like TUN devices will not have an address, and while
iterating over ifaddrs, its ifa_addr will be NULL. This patch fixes a crash when
starting ovs-vswitchd on a system with such a device.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Fixes: a8704b502785 ("tunneling: Handle multiple ip address for given device.")
Cc: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>