[tsan] Fix a crash with dispatch_source_set_cancel_handler(NULL) on OS X

[kubamracek]

Summary

We need to handle the case when handler is NULL in dispatch_source_set_cancel_handler and similar interceptors.

compiler-rt/trunk/lib/tsan/rtl/tsan_libdispatch_mac.cc

TSAN_INTERCEPTOR(void, dispatch_group_notify_f, dispatch_group_t group,
   Release(thr, pc, (uptr)group);
   REAL(dispatch_group_notify_f)(group, q, new_context,
                                 dispatch_callback_wrap);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_event_handler,
                  dispatch_source_t source, dispatch_block_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_event_handler, source, handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_event_handler)(source, nullptr);
   dispatch_block_t new_handler = ^(void) {
     {
       SCOPED_INTERCEPTOR_RAW(dispatch_source_set_event_handler_callback);
       Acquire(thr, pc, (uptr)source);
     }
     handler();
   };
   Release(thr, pc, (uptr)source);
   REAL(dispatch_source_set_event_handler)(source, new_handler);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_event_handler_f,
                  dispatch_source_t source, dispatch_function_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_event_handler_f, source, handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_event_handler)(source, nullptr);
   dispatch_block_t block = ^(void) {
     handler(dispatch_get_context(source));
   };
   WRAP(dispatch_source_set_event_handler)(source, block);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_cancel_handler,
                  dispatch_source_t source, dispatch_block_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_cancel_handler, source, handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_cancel_handler)(source, nullptr);
   dispatch_block_t new_handler = ^(void) {
     {
       SCOPED_INTERCEPTOR_RAW(dispatch_source_set_cancel_handler_callback);
       Acquire(thr, pc, (uptr)source);
     }
     handler();
   };
   Release(thr, pc, (uptr)source);
   REAL(dispatch_source_set_cancel_handler)(source, new_handler);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_cancel_handler_f,
                  dispatch_source_t source, dispatch_function_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_cancel_handler_f, source,
                           handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_cancel_handler)(source, nullptr);
   dispatch_block_t block = ^(void) {
     handler(dispatch_get_context(source));
   };
   WRAP(dispatch_source_set_cancel_handler)(source, block);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_registration_handler,
                  dispatch_source_t source, dispatch_block_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_registration_handler, source,
                           handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_registration_handler)(source, nullptr);
   dispatch_block_t new_handler = ^(void) {
     {
       SCOPED_INTERCEPTOR_RAW(dispatch_source_set_registration_handler_callback);
       Acquire(thr, pc, (uptr)source);
     }
     handler();
   };
   Release(thr, pc, (uptr)source);
   REAL(dispatch_source_set_registration_handler)(source, new_handler);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_source_set_registration_handler_f,
                  dispatch_source_t source, dispatch_function_t handler) {
   SCOPED_TSAN_INTERCEPTOR(dispatch_source_set_registration_handler_f, source,
                           handler);
+  if (handler == nullptr)
+    return REAL(dispatch_source_set_registration_handler)(source, nullptr);
   dispatch_block_t block = ^(void) {
     handler(dispatch_get_context(source));
   };
   WRAP(dispatch_source_set_registration_handler)(source, block);
 }
 
 TSAN_INTERCEPTOR(void, dispatch_apply, size_t iterations,
                  dispatch_queue_t queue, void (^block)(size_t)) {