libventi: fix segfault in vtgetreq

Don't call vtlog() when _vtqrecv() returns
nil on queue hangup.

This change has been reviewed and merged
in plan9port.

Reference: /n/sources/patch/applied/libventi-vtgetreq-vtlog
Date: Fri Aug  3 21:17:11 CES 2012
Signed-off-by: djc@9grid.fr

--- /sys/src/libventi/server.c	Fri Aug  3 21:16:59 2012
+++ /sys/src/libventi/server.c	Fri Aug  3 21:16:57 2012
@@ -180,7 +180,8 @@
 	VtReq *r;
 	
 	r = _vtqrecv(srv->q);
-	vtlog(VtServerLog, "<font size=-1>%T %s:</font> vtgetreq %F<br>\n", ((VtSconn*)r->sc)->c->addr, &r->tx);
+	if (r != nil)
+		vtlog(VtServerLog, "<font size=-1>%T %s:</font> vtgetreq %F<br>\n", ((VtSconn*)r->sc)->c->addr, &r->tx);
 	return r;
 }