Fix SIGSEGV for netwowrk --device= which is not found (#697432)
We ended up with netDev_set == 1 and netDev == NULL in the case
which resulted in SIGSEGV later. The patch sets netDev to value
of --device option which leads to asking in UI as in other similar
cases.
Resolves: rhbz#697432
The patch is tested on master, and is not relevant for F15.