mongodump panics if collection doesn't exist

If mongodump is run with a database/collection that doesn't exist, it panics with a nil pointer dereference. From a quick look, I think this is because GetCollectionInfo on prepare.go line 227 can return nil if the collection isn't found, but this isn't checked anywhere. A similar problem was addressed in the mongoexport port to the new driver by checking if the collection exists before exporting any documents.