Description:
if i manually remove a mapping done with cryptsetup (at boot time by /etc/crypttab), rc.shutdown try anyway to remove that non-existent mapping

for example sda2 mappings created at boot time.
then there is a /dev/mapper/sda2.
$ cryptsetup luksClose sda2
/dev/mapper/sda2 is gone
but rc.shutdown try to remove it anyway, causing an error

proposed patch is just to check if mapping is still there before trying to remove it with a test
if [ -b /dev/mapper/$1 ]

alternate test could be
if cryptsetup status $1 &>/dev/null

cons:
- the error is harmfull except it pollutes the shutdown screen
- it is not THE normal way of things to manually remove a crypted partition automatically mounted

pros:
- it's just a simple test