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.

src/net_processing.cpp

@@ -3006,6 +3006,7 @@ void PeerLogicValidation::ConsiderEviction(CNode *pto, int64_t time_in_seconds)
                 LogPrintf("Disconnecting outbound peer %d for old chain, best known block = %s\n", pto->GetId(), state.pindexBestKnownBlock != nullptr ? state.pindexBestKnownBlock->GetBlockHash().ToString() : "<none>");
                 pto->fDisconnect = true;
             } else {
+                assert(state.m_chain_sync.m_work_header);
                 LogPrint(BCLog::NET, "sending getheaders to outbound peer=%d to verify chain work (current best known block:%s, benchmark blockhash: %s)\n", pto->GetId(), state.pindexBestKnownBlock != nullptr ? state.pindexBestKnownBlock->GetBlockHash().ToString() : "<none>", state.m_chain_sync.m_work_header->GetBlockHash().ToString());
                 connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETHEADERS, chainActive.GetLocator(state.m_chain_sync.m_work_header->pprev), uint256()));
                 state.m_chain_sync.m_sent_getheaders = true;