Andrew Zaborowski authored Aug 13, 2018 and Thomas Haller committed Sep 05, 2018

Instead of passing the return values to g_variant_get_string or
g_variant_boolean and then checking the return value of that call,
add wrappers that first check's whether the variant is non-NULL
and of the right type.
g_variant_get_string doesn't allow a NULL parameter and will also never
return NULL according to the docs.

For the State property we assume a state "unknown" and emit a warning
if the property can't be read, "unknown" is also a string in IWD itself
which would be returned if something went really wrong.  In any case
this shouldn't happen.

[thaller@redhat.com: fix missing initialization of nm_auto() variable
  interfaces.]