kv_flat_btree_async.cc: fix resource leak

Call AioCompletion::release() if the completion is no longer
needed to free the resources.

CID 727976 Resource leak (CWE-404)
CID 727977 Resource leak (CWE-404)

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ceph-monstore-tool.cc: check if open() was successful

Should fix: "fd" is passed to a parameter that cannot be negative.

CID 1019566 Improper use of negative value (NEGATIVE_RETURNS, CWE-394)

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>