event/SocketMonitor: don't cancel if OnSocketReady() returns false

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

src/event/BufferedSocket.cxx

@@ -110,15 +110,9 @@ BufferedSocket::OnSocketReady(unsigned flags) noexcept
 	if (flags & READ) {
 		assert(!input.IsFull());
 
-		if (!ReadToBuffer())
+		if (!ReadToBuffer() || !ResumeInput())
 			return false;
 
-		if (!ResumeInput())
-			/* we must return "true" here or
-			   SocketMonitor::Dispatch() will call
-			   Cancel() on a freed object */
-			return true;
-
 		if (!input.IsFull())
 			ScheduleRead();
 	}