storage_proxy: fix crash during background read repair
storage_proxy: fix crash during background read repair

Lazy digest calculation code introduced a bug in background read repair.
The problem is that digest_read_resolver::resolve() destroys one data
result (it is moved to a caller to be sent as a reply), so during
background digest match there is no value to calculate a digest from.
Copying data to the caller would be most elegant solution, but also
slowest one, so lets just treat the case where there is only one
target queried and skip digest calculation in this case since we know
digest_match() will do nothing.

scylladb/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,kangkot/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,duarten/scylla,kangkot/scylla,kangkot/scylla,avikivity/scylla