res_calendar: Protect channel when adding datastore.

This change adds a missing channel lock when adding a datastore
to a channel.
........

Merged revisions 404135 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@404136 f38db490-d61c-443f-a65b-d21fe96a405b