Check for shutdown in pauseRsBgSyncProducer failpoint

[2016/11/07 05:13:54.593] Thread 39 (Thread 0x7f530dc42700 (LWP 9183)):

[2016/11/07 05:13:54.593] #0  0x00007f5313c792fd in pthread_join () from /lib64/libpthread.so.0

[2016/11/07 05:13:54.593] #1  0x00007f53187c7d07 in std::thread::join() ()

[2016/11/07 05:13:54.593] #2  0x00007f53176d98f6 in mongo::repl::BackgroundSync::join(mongo::OperationContext*) ()

[2016/11/07 05:13:54.593] #3  0x00007f53177a5307 in mongo::repl::ReplicationCoordinatorExternalStateImpl::_stopDataReplication_inlock(mongo::OperationContext*, std::unique_lock<std::mutex>*) ()

[2016/11/07 05:13:54.594] #4  0x00007f53177a6b3d in mongo::repl::ReplicationCoordinatorExternalStateImpl::shutdown(mongo::OperationContext*) ()

[2016/11/07 05:13:54.594] #5  0x00007f53177b86aa in mongo::repl::ReplicationCoordinatorImpl::shutdown(mongo::OperationContext*) ()

[2016/11/07 05:13:54.594] #6  0x00007f5316f9d9d7 in shutdownTask() ()

[2016/11/07 05:13:54.594] #7  0x00007f5317cd3ea2 in mongo::(anonymous namespace)::runTasks(std::stack<std::function<void ()>, std::deque<std::function<void ()>, std::allocator<std::function<void ()> > > >) [clone .constprop.62] ()

[2016/11/07 05:13:54.594] #8  0x00007f5316f2dede in mongo::shutdown(mongo::ExitCode) ()

[2016/11/07 05:13:54.594] #9  0x00007f5317d54a9e in mongo::(anonymous namespace)::signalProcessingThread() ()

...

[2016/11/07 05:13:54.644] Thread 3 (Thread 0x7f52f4d02700 (LWP 9397)):

[2016/11/07 05:13:54.644] #0  0x00007f5317cd53c8 in mongo::FailPoint::shouldFailCloseBlock() ()

[2016/11/07 05:13:54.644] #1  0x00007f53176dc153 in mongo::repl::BackgroundSync::_produce(mongo::OperationContext*) ()

[2016/11/07 05:13:54.644] #2  0x00007f53176de1fd in mongo::repl::BackgroundSync::_runProducer() ()

[2016/11/07 05:13:54.644] #3  0x00007f53176de42a in mongo::repl::BackgroundSync::_run() ()

...