MFC r215637:

dispatch_add_command:
Modify the logic so there's only one exit point instead of two.
Only insert valid (non-NULL) values into the queue.

dispatch_free_command:
Ensure that item is not NULL before removing it from the queue and
dereferencing the pointer.
NULL out free'd pointers to catch any use-after-free bugs.

PR: bin/146855
Submitted by: gcooper