libventi: fix segfault in vtgetreq

Don't call vtlog() when _vtqrecv() returns
nil on queue hangup, as reported on
http://bitbucket.org/rsc/plan9port/issue/102/

R=rsc
http://codereview.appspot.com/6373046

src/libventi/server.c

@@ -182,6 +182,7 @@ vtgetreq(VtSrv *srv)
 	VtReq *r;
 	
 	r = _vtqrecv(srv->q);
+	if (r != nil)
 		vtlog(VtServerLog, "<font size=-1>%T %s:</font> vtgetreq %F<br>\n", ((VtSconn*)r->sc)->c->addr, &r->tx);
 	return r;
 }