Handle RPCs in libventi when not connected to a Venti server.

This keeps the behavior more consistant with the old libventi.
The old libventi handle this case properly in the function vtRPC
(/sys/src/liboventi/client.c).

It has been reviewed and merged in plan9port.

Reference: /n/sources/patch/applied/libventi-rpc-discon
Date: Wed Jan 18 10:10:36 CET 2012
Signed-off-by: djc@9grid.fr

--- /sys/src/libventi/rpc.c	Wed Jan 18 10:10:21 2012
+++ /sys/src/libventi/rpc.c	Wed Jan 18 10:10:17 2012
@@ -37,6 +37,12 @@
 	uchar tag, buf[2], *top;
 	Rwait *r, *rr;
 
+	if(z == nil){
+		werrstr("not connected");
+		packetfree(p);
+		return nil;
+	}
+
 	/* must malloc because stack could be private */
 	r = vtmallocz(sizeof(Rwait));