sstables: fix use-after-free in read_simple()
sstables: fix use-after-free in read_simple()

`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: <9e7c5c5c520bf3f8dec95b36d57d68d381c1d130@scylladb.com>

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