Check for source activity's task presence before referencing.

ActivityStarter guards in all other spots for a null task in the
source activity, which can be the case if said activity is finishing.

This changelist adds the same check to the code when processing a
finishing source.

Change-Id: I78c284f45d96348767d06bb6ce0d78d6b77cb0ad
Fixes: 37428712
Test: manual testing & code inspection
Test: go/wm-smoke