sd-journal: always put verified object into the chain cache

sd-journal: always put verified object into the chain cache

Let's consider the case that
- the first array contains valid entries,
- all entries in the second array are corrupted.

Then, when we are going to upwards, and a call of generic_array_bisect()
matches the last entry of the first array, then the second array was
cached with last_index == UINT64_MAX, instead of the first array with
its last entry.
Hence, when generic_array_bisect() is called next time, the function call
of test() always fail. So, the cache entry is mostly meaningless.

Let's always store valid cache entry.