Move lock acquire/release functions in task deque cleanup code

The original locations can be reached without initializing the lock variable
(td_deque_lock), so it is potentially unsafe.  It is guaranteed that the lock
is initialized if the deque (td_deque) is not NULL, and lock functions can be
safely called.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D36017

llvm-svn: 309875