Fix tcpbench libevent segfault

[Christiano F. Haesbaert]

Hi there, this fixes a segfault seen by sthen@ which I could only
reproduce yesterday. I had fogotten to event_del() a persistent event. 

Index: tcpbench.c
===================================================================
RCS file: /cvs/src/usr.bin/tcpbench/tcpbench.c,v
retrieving revision 1.22
diff -d -u -p -w -r1.22 tcpbench.c
--- tcpbench.c  21 Jun 2011 17:31:07 -0000      1.22
+++ tcpbench.c  20 Aug 2011 06:34:06 -0000
@@ -642,6 +642,7 @@ again:
        } else if (n == 0) {
                if (ptb->vflag)
                        fprintf(stderr, "%8d closed by remote end\n", sc->fd);
+               event_del(&sc->ev);
                close(sc->fd);
                TAILQ_REMOVE(&sc_queue, sc, entry);
                free(sc);