event/Loop: remove bogus "!quit" assertion

Commit 1f11959 allowed modifying the "quit" attribute from any thread,
and thus the assertion may fail spuriously.  This assertion is too
strict for the relaxed use of "quit".  Let's remove it and move the
"quit" check to before the SockedMonitor::Dispatch() call.

src/event/Loop.cxx

@@ -165,17 +165,15 @@ EventLoop::Run()
 
 		now_ms = ::MonotonicClockMS();
 
-		assert(!quit);
-
 		/* invoke sockets */
 		for (int i = 0; i < poll_result.GetSize(); ++i) {
 			auto events = poll_result.GetEvents(i);
 			if (events != 0) {
-				auto m = (SocketMonitor *)poll_result.GetObject(i);
-				m->Dispatch(events);
-
 				if (quit)
 					break;
+
+				auto m = (SocketMonitor *)poll_result.GetObject(i);
+				m->Dispatch(events);
 			}
 		}