We store the per-shard chunk count in a uint64_t vector
global_offset, and then convert the counts to offsets with
a prefix sum:
```c++
// [1, 2, 3, 0] --> [0, 1, 3, 6]
std::exclusive_scan(global_offset.begin(), global_offset.end(), global_offset.begin(), 0, std::plus());
```
However, std::exclusive_scan takes the accumulator type from the
initial value, 0, which is an int, instead of from the range being
iterated, which is of uint64_t.
As a result, the prefix sum is computed as a 32-bit integer value. If
it exceeds 0x8000'0000, it becomes negative. It is then extended to
64 bits and stored. The result is a huge 64-bit number. Later on
we try to find an sstable with this chunk and fail, crashing on
an assertion.
An example of the failure can be seen here: https://godbolt.org/z/6M8aEbo57
The fix is simple: the initial value is passed as uint64_t instead of int.
Fixes https://github.com/scylladb/scylladb/issues/27417Closesscylladb/scylladb#27418
9696ee64d0