samples: net: zoap_server: Check IPv6 address add return value

When unicast IPv6 address is added to network interface, it is
possible that the return value is NULL (if all the address slots
are already occupied).

Coverity-CID: 157607

Jira: ZEP-2467

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>