before this change, when running into a zero chunk_len, scylla
crashes with `assert(chunk_size != 0)`. but we can do better than
printing a backtrace like:
```
scylla: sstables/compress.cc:158: void
sstables::compression::segmented_offsets::init(uint32_t): Assertion `chunk_size != 0' failed.
```
so, in this change, a `malformed_sstable_exception` is throw in place
of an `assert()`, which is supposed to verify the programming
invariants, not for identifying corrupted data file.
Fixes#15265
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#15264
1ed894170c