the test at fs.c:1233 is too late: mb->d->qid.path
will already have been dereferenced.  i've had
fs break due to this.

Reference: /n/sources/patch/applied/upas-fs-nil-test
Date: Tue Feb  8 06:57:28 CET 2005

--- /sys/src/cmd/upas/fs/fs.c	Tue Feb  8 06:55:23 2005
+++ /sys/src/cmd/upas/fs/fs.c	Tue Feb  8 06:55:19 2005
@@ -1229,8 +1229,8 @@
 				continue;
 
 			qlock(mb);
-			if(d->qid.path != mb->d->qid.path
-			   || mb->d && d->qid.vers != mb->d->qid.vers){
+			if(mb->d && (d->qid.path != mb->d->qid.path
+			   || d->qid.vers != mb->d->qid.vers)){
 				free(d);
 				break;
 			}