lib9p: fix zero msize abort() due to unknown version (thanks kivik)

kivik wrote:

I've found a nasty bug in lib9p handling of Tversion
messages, where an invalid version string in the request
leads to servers abort()ing the spaceship.

To reproduce:
	; ramfs -S ram
	; aux/9pcon /srv/ram
	Tversion ~0 DIE

The issue lies in sversion() where in case an invalid
version string is received we respond right away with
ofcall.version="unknown"; however, we fail to set the
ofcall.msize, which at this point is cleared to 0.  This
causes the convS2M call in respond() to fail and abort being
called.

sys/src/lib9p/srv.c

@@ -172,6 +172,7 @@ sversion(Srv *srv, Req *r)
 	}
 	if(strncmp(r->ifcall.version, "9P", 2) != 0){
 		r->ofcall.version = "unknown";
+		r->ofcall.msize = 256;
 		respond(r, nil);
 		return;
 	}