engine: Fix coverity warnings (NULL_RETURNS)

Although the warned scenario is unlikely to occur,
it would be advisable to terminate network configuration
base on network labels if an external event (i.e. refresh
host capabilities) will cause an existing labeled nic to
disappear. This is a rare race, but can happen.

Change-Id: Iea07be387e71f0dfb8d9a78d8e03c504095c2062
Signed-off-by: Moti Asayag <masayag@redhat.com>