[tsan] Only Acquire/Release GCD queues if they're not NULL

While it's usually a bug to call GCD APIs, such as dispatch_after, with NULL as a queue, this often "somehow" works and TSan should maintain binary compatibility with existing code. This patch makes sure we don't try to call Acquire and Release on NULL queues, and add one such testcase for dispatch_after.

Differential Revision: https://reviews.llvm.org/D31355

llvm-svn: 298820