results-view: Abort early if messing with empty vector
results-view: Abort early if messing with empty vector

The .get_last_partition_and_clustering_key() method gets
the last partition from the on-board vector of partitions.
The vector in question is assumed not to be empty, but if
this assumption breaks, the result will look like memory
corruption (docs say that accessing empty's vector back()
results in undefined behavior).

tests: unit(dev)

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20200921122948.20585-1-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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