ipconfig: Do not pass a NULL pointer to D-Bus

connman_inet_ifname() may return a NULL pointer e.g. if the interface
does not exist in the kernel anymore but connman isn't yet aware of
that. Passing a NULL pointer to D-Bus may result in an abort, so check
the return value.

[connman] ipconfig: Do not pass a NULL pointer to D-Bus
Merge pull request #121 from hmallat/connman-inet-ifname-null-check

ipconfig: Do not pass a NULL pointer to D-Bus