Summary:
* db/range_del_aggregator.cc (AddTombstone): Avoid a potential
use-after-move bug. The original code would both use and move
`tombstone` in a context where the order of those operations is
not specified.  The fix is to perform the use on a new, preceding
statement.

Author: meyering
Closes https://github.com/facebook/rocksdb/pull/2796

Differential Revision: D5721163

Pulled By: ajkr

fbshipit-source-id: a1d328d6a77a17c6425e8069860a202e615e2f48