removable devices can may return 0 sectors in scsionline when they have empty
media.  prevent infinite loops.  ahci+sata lg super multi dvd rewriter do this, e.g.

Notes:
Wed Aug 8 18:50:35 EDT 2007 geoff
    applied in slightly different form.


Reference: /n/sources/patch/applied/scsionline-removeable
Date: Wed Aug  8 16:43:35 CES 2007
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/9/pc/sdscsi.c	Wed Aug  8 16:39:11 2007
+++ /sys/src/9/pc/sdscsi.c	Wed Aug  8 16:39:08 2007
@@ -215,8 +215,10 @@
 			break;
 		case 0:
 			unit->sectors = (p[0]<<24)|(p[1]<<16)|(p[2]<<8)|p[3];
-			if(unit->sectors == 0)
-				continue;
+			if(unit->sectors == 0){
+				ok = 1;
+				break;
+			}
 			/*
 			 * Read-capacity returns the LBA of the last sector,
 			 * therefore the number of sectors must be incremented.