libventi: fix segfault in vtreadpacket when not connected

This problem happens especially in fossil/view when vtread
can be called even if not connected to venti.

The old libventi handle this case properly in the function vtRPC, which is called by vtReadPacket (client.c).

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/5484044
1 file changed