dbus_new_handlers: Don't send NULL to dbus_message_new_error

dbus_new_handlers: Don't send NULL to dbus_message_new_error

The new DBus API helper function wpas_dbus_error_unknown_error
function can be called as a result of a failure within internal
getter calls, which will call this function with a NULL message
parameter.  However, dbus_message_new_error looks very unkindly
(i.e, abort()) on a NULL message, so in this case, we should not
call it.

I've observed this course of events during a call to
wpas_dbus_getter_bss_wpa with a faileld parse of the IE parameter.
We got here through a call to fill_dict_with_properties which
explicitly calls getters with a NULL message parameter.  Judging
from the way it is called, this could easily occur if an AP sends
out a malformed (or mis-received) probe response.  I usually run
into this problem while driving through San Francisco, so I'm
exposed to any number of base stations along this path.