Don't free stack variables

couch-kvstore.cc:1644:27: warning: attempt to free a non-heap object 'ext_meta' [-Wfree-nonheap-object]

Change-Id: Iff414c633af0976d8036ad58b42ee24a638f6387
Reviewed-on: http://review.couchbase.org/38235
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>