device: Fix NULL pointer dereference during GATT service discovery

device: Fix NULL pointer dereference during GATT service discovery

If reverse service discovery for GATT is triggered,
device_browse_primary() is called with a NULL "conn" parameter. This fix
is based on similar code found in device_browse_sdp().

This fixes errors like:

process 871: arguments to dbus_connection_ref() were incorrect,
assertion "connection != NULL" failed in file dbus-connection.c line
2549.
This is normally a bug in some application using the D-Bus library.
process 871: arguments to dbus_connection_get_object_path_data() were
incorrect, assertion "connection != NULL" failed in file
dbus-connection.c line 5639.
This is normally a bug in some application using the D-Bus library.
process 871: arguments to dbus_connection_register_object_path() were
incorrect, assertion "connection != NULL" failed in file
dbus-connection.c line 5461.
This is normally a bug in some application using the D-Bus library.