virGDBusBusInit: Properly check for error when looking up D-Bus address

virGDBusBusInit: Properly check for error when looking up D-Bus address

The virGDBusBusInit is supposed to return a reference to
requested bus type (system/session) or, if non-shared bus is
requested then create a new bus of the type. As an argument, it
gets a double pointer to GError which is passed to all g_dbus_*()
calls which allocate it on failure. Pretty standard approach.
However, since it is a double pointer we must dereference the
first level to see if the value is NULL. IOW:

  if (*error)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>