MINOR: poller: do not call the IO handler if the FD is not active

For now this almost never happens but with subsequent patches it will
become more important not to uselessly call the I/O handlers if the FD
is not active.
diff --git a/include/proto/fd.h b/include/proto/fd.h
index 8dd64a5..a507485 100644
--- a/include/proto/fd.h
+++ b/include/proto/fd.h
@@ -358,7 +358,7 @@
 	if (fdtab[fd].ev & (FD_POLL_OUT | FD_POLL_ERR))
 		fd_may_send(fd);
 
-	if (fdtab[fd].iocb)
+	if (fdtab[fd].iocb && fd_active(fd))
 		fdtab[fd].iocb(fd);
 
 	ti->flags &= ~TI_FL_STUCK; // this thread is still running