LibCore: Fix errors when compiling LibCore using clang instead of gcc

Author: https://github.com/Pagghiu
Commit: aab412bd85
Pull-request: https://github.com/SerenityOS/serenity/pull/841
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Libraries/LibCore/CEventLoop.cpp

@@ -241,8 +241,9 @@ void CEventLoop::pump(WaitMode mode)
 
     for (int i = 0; i < events.size(); ++i) {
         auto& queued_event = events.at(i);
+#ifndef __clang__
         ASSERT(queued_event.event);
-
+#endif
         auto* receiver = queued_event.receiver.ptr();
         auto& event = *queued_event.event;
 #ifdef CEVENTLOOP_DEBUG