Check if the command buffer has been allocated before we free it.  Fixes a possible page fault trap while device detach.
Check if the command buffer has been allocated before we free it.  Fixes
a possible page fault trap while device detach.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars