redis: fix use-after-free crash in "exists" command
redis: fix use-after-free crash in "exists" command

A missing "&" caused the key stored in a long-living command to be copied
and the copy quickly freed - and then used after freed.
This caused the test test_strings.py::test_exists_multiple_existent_key for
this feature to frequently crash.

Fixes #6469

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200823190141.88816-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla