When using libdisk functions with files, not real or partfs(8) disks, chs fields in the Disk structure are not filled. 
Later on, partitiongeometry() will try to calculate the cylinders with
	disk->c = disk->secs / (disk->h*disk->s)
which may result in a division by zero.
This patch just checks for empty chs.
.

Reference: /n/sources/patch/applied/diskgeometry-nochs
Date: Thu Nov 19 12:32:56 CET 2009
Signed-off-by: iru.muzgo@gmail.com

--- /sys/src/libdisk/disk.c	Thu Nov 19 12:25:55 2009
+++ /sys/src/libdisk/disk.c	Thu Nov 19 12:25:49 2009
@@ -63,6 +63,9 @@
 	uchar buf[512];
 	Table *t;
 
+	if(disk->c == 0 || disk->h == 0 || disk->s == 0)
+		return -1;
+
 	t = (Table*)(buf + Toffset);
 
 	/*