Explicitly state assumption that `state.m_chain_sync.m_work_header != nullptr` in `ConsiderEviction(…)`.

Static analyzer (and humans!) will see the null-check in ...

```
else if (state.m_chain_sync.m_timeout == 0 || (state.m_chain_sync.m_work_header != nullptr && ...
```

... and infer that `state.m_chain_sync.m_work_header` might be set to `nullptr` when reaching `else if (state.m_chain_sync.m_timeout > 0 && time_in_seconds > state.m_chain_sync.m_timeout)` and thus flag `state.m_chain_sync.m_work_header->GetBlockHash().ToString()` as a potential null pointer dereference.

This commit makes the tacit assumption of `state.m_chain_sync.m_work_header != nullptr` explicit.

Code introduced in 5a6d00c6defc587e22c93e63029fdd538ce8858d ("Permit disconnection of outbound peers on bad/slow chains") which was merged into master four days ago.

Friendly ping @sdaftuar :-)

make check
test_runner.py