Currently the function calls boost::partial_sort with a middle
iterator that might be out of bound and cause undefined behavior.
Check the vector size, and do a partial sort only if its longer
than `max_sstables`, otherwise sort the whole vector.
Fixesscylladb/scylladb#20608
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closesscylladb/scylladb#20609