res_calendar: Protect channel when adding datastore.

This change adds a missing channel lock when adding a datastore
to a channel.
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Merged revisions 404135 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 404136 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@404137 f38db490-d61c-443f-a65b-d21fe96a405b