missing quotes confuses try_snap

Binary package hint: casper

try_snap is looking for a cow_device but when it does not find one, and $dev is "" (empty) it falsely starts treating the empty device as a block device, with a bunch of follow-up errors.

Intuitive or not, [ -b ] is true but [ -b "" ] is false.

casper-log:
+ dev=
+ [ -b ]
+ echo
+ grep -qs loop
+ snapdev=
+ do_snap_copy /root ROOT
+ fromdev=
+ todir=/root
+ snap_type=ROOT
+ fs_size used
+ dev=
+ mountp=
+ used=used
+ [ -z ]
+ where_is_mounted
+ device=
+ grep -q ^ /proc/mounts
+ return 1
+ mountp=
+ [ 1 -gt 0 ]
+ mountp=/mnt/tmp_fs_size
+ mkdir -p /mnt/tmp_fs_size
+ get_fstype
+ local FSTYPE
+ local FSSIZE
/init: line 1: can't open : no such file
+ fstype
+ eval
+ [ != unknown ]
+ echo
+ return 0
+ mount -t -o ro /mnt/tmp_fs_size

ProblemType: Bug
Architecture: i386
Date: Sat Sep 5 10:54:00 2009
DistroRelease: Ubuntu 9.10
Package: casper 1.189
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-9.29-generic
SourcePackage: casper
Uname: Linux 2.6.31-9-generic i686