MEDIUM: polling: prepare to call the iocb() function when defined.

We will need this to centralize I/O callbacks. Nobody sets it right
now so the code should have no impact.
diff --git a/src/ev_epoll.c b/src/ev_epoll.c
index 482992e..589932f 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -260,15 +260,20 @@
 			if (!fdtab[fd].owner)
 				continue;
 			if (fdtab[fd].ev & (FD_POLL_IN|FD_POLL_HUP|FD_POLL_ERR))
-				fdtab[fd].cb[DIR_RD].f(fd);
+				if (fdtab[fd].cb[DIR_RD].f)
+					fdtab[fd].cb[DIR_RD].f(fd);
 		}
 
 		if ((fd_evts[FD2OFS(fd)] >> FD2BIT(fd)) & DIR2MSK(DIR_WR)) {
 			if (!fdtab[fd].owner)
 				continue;
 			if (fdtab[fd].ev & (FD_POLL_OUT|FD_POLL_ERR|FD_POLL_HUP))
-				fdtab[fd].cb[DIR_WR].f(fd);
+				if (fdtab[fd].cb[DIR_WR].f)
+					fdtab[fd].cb[DIR_WR].f(fd);
 		}
+
+		if (fdtab[fd].iocb && fdtab[fd].owner && fdtab[fd].ev)
+			fdtab[fd].iocb(fd);
 	}
 }