Avoid NPE around mReusedTask of ActivityStarter

Symptom:
System crash by NPE is observed on ActivityStarter
during phone boot-up.

Detail and sample:
ActivityStarter has possibility to be called itself recursively
by calling TaskRecord$performClearTaskLocked().
Then class variable of mReusedTask is initialized to null
after the call, because the recursive call isn't guarded by
synchronized block.
Then NPE crash occurs on next statement.

Solutions:
Avoid to use the class variable on null possibility statement.

Bug: 32361138

Change-Id: Iaea3e066a6f7134fcae4338ff864bb236241194c