handover_fsm: send_handover_performed(): check lchan != NULL
The target lchan is expected to be valid on HO_RESULT_OK, but it's
still safer to check it against NULL before dereferencing.
Change-Id: Iee734c98dc79681808e7d75360c208a941cc6821
Related: 107391d8c ("handover_fsm: fix send_handover_performed() using wrong lchan")
