kv_flat_btree_async.cc: release AioCompletion before leave the loop

NOTE: not 100% sure this is the correct fix, so please review carefully.
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CID 727982 (#1 of 1): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "aioc" going out of scope leaks the storage
  it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>