Add missing NULL pointers assignments

Each pointer that was freed by one of the following functions should
be reset to NULL:
 - config_free()
 - data_dispatcher_free()
 - fixed_queue_free()
 - list_free()

Bug: 29421693
Change-Id: Ie55a04ed986393994564edcf872d7413b0767a85
(cherry picked from commit 5b2a7db8)