cntl: string passthrough should pass a void*, not void**

This fixes an issue where a string cntl would pass a pointer to a string
rather than a string itself to the cntl handler.

Change-Id: Ica78d8682448efd4b81caf32de2055b944ef6506
Reviewed-on: http://review.couchbase.org/39930
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>