Attempting to call advance_to() on the index, after it is positioned at EOF, can result in an assert failure, because the operation results in an attempt to move backwards in the index-file (to read the last index page, which was already read). This only happens if the index cache entry belonging to the last index page is evicted, otherwise the advance operation just looks-up said entry and returns it. To prevent this, we add an early return conditioned on eof() to all the partition-level advance-to methods.
A regression unit test reproducing the above described crash is also added.
Fixes: #10403Closes#10491
* github.com:scylladb/scylla:
sstables/index_reader: short-circuit fast-forward-to when at EOF
test/lib/random_schema: add a simpler overload for fixed partition count
94f677b790