libgdbus: Return null when dbus connection fails

libgdbus: Return null when dbus connection fails

When libgdbus fails to get bus for one of global busses (system, session)
it still returns vaild pointer.  It's because global busses are cached in
g_dh[2] which always exist.

This commit changes the get_connection function to return null for all the
checks like following to detect and fail in the case where getting bus was
unsuccessfull (ie. dbus-daemon isn't running in early boot/special targets)

    dh = _dbus_handle_get_default_connection();
    if (!dh) {
        _E("failed to get default connection, bustype:%d", (int)dbus_handle_get_default_bus_type());
        ...

Change-Id: If85fe8e6cb7a5f95618df8f7f3fefa923e3e271a