`r` is moved-from, and later captured in a different lambda. The compiler may
choose to move and perform the other capture later, resulting in a use-after-free.
Fix by copying `r` instead of moving it.
Discovered by sstable_test in debug mode.
Message-Id: <20170702082546.20570-1-avi@scylladb.com>
(cherry picked from commit 07b8adce0e)