Explicitly state assumption that state.m_chain_sync.m_work_header != nullptr in ConsiderEviction
Static analyzer (and humans!) will see ...
```
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,
and thus flag `state.m_chain_sync.m_work_header->GetBlockHash().ToString()`
as a potential null pointer dereference.
This commit makes the tacit assumption (m_work_header != nullptr) explicit.
Code introduced in 5a6d00 ("Permit disconnection of outbound peers on
bad/slow chains") which was merged into master four days ago.