Fixed panic on task cancel
Fixed panic on task cancel

This commit fixes the ec2 driver where if a task is running and
it is canceled by the operator, it would typically result in
a panic.  This was caused since the volume attachment would return
nil and it would still be checked for status.

emccode/rexray,cduchesne/rexray,oppodeldoc/rexray,cduchesne/rexray,thecodeteam/rexray,akutz/rexray,oppodeldoc/rexray,emccode/rexray,codedellemc/rexray,codedellemc/rexray,akutz/rexray,oppodeldoc/rexray,oppodeldoc/rexray,akutz/rexray,cduchesne/rexray,emccode/rexray,clintonskitson/rexray,thecodeteam/rexray,thecodeteam/rexray,codedellemc/rexray,clintonskitson/rexray,cduchesne/rexray,codedellemc/rexray,clintonskitson/rexray,oppodeldoc/rexray,thecodeteam/rexray,cduchesne/rexray,cduchesne/rexray,thecodeteam/rexray,oppodeldoc/rexray,codedellemc/rexray,emccode/rexray,akutz/rexray,akutz/rexray,codedellemc/rexray,clintonskitson/rexray,thecodeteam/rexray,akutz/rexray