fix overflow of up->genbuf.

charles suggestion is better than what i did, which was half in the previous
patch.  i had redefined up->genbuf to be char genbuf[ERRMAX], and
that was not in the patch.

Reference: /n/patches.lsub.org/patch/devenvovf
Date: Mon Oct 29 15:41:49 CET 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/port/devenv.c	Thu Apr 12 12:26:28 2012
+++ /sys/src/nix/port/devenv.c	Mon Oct 29 14:44:19 2012
@@ -8,6 +8,7 @@
 enum
 {
 	Maxenvsize = 16300,
+	Maxename = sizeof up->genbuf-1,
 };
 
 static Egrp	*envgrp(Chan *c);
@@ -146,6 +147,8 @@
 
 	if(c->qid.type != QTDIR)
 		error(Eperm);
+	if(strlen(name) > Maxename)
+		error("name too long");
 
 	omode = openmode(omode);
 	eg = envgrp(c);