Linkgraph: Avoid attempting to merge/apply job when cleaning schedule.

This can cause crashes when switching savegame.
Move job merge/apply out of destructor into separate function.

src/linkgraph/linkgraphjob.cpp

@@ -82,15 +82,19 @@ void LinkGraphJob::JoinThread()
 }
 
 /**
- * Join the link graph job and destroy it.
+ * Join the link graph job thread, if not already joined.
  */
 LinkGraphJob::~LinkGraphJob()
 {
 	this->JoinThread();
+}
 
-	/* Don't update stuff from other pools, when everything is being removed.
-	 * Accessing other pools may be invalid. */
-	if (CleaningPool()) return;
+/**
+ * Join the link graph job thread, then merge/apply it.
+ */
+void LinkGraphJob::FinaliseJob()
+{
+	this->JoinThread();
 
 	/* Link graph has been merged into another one. */
 	if (!LinkGraph::IsValidID(this->link_graph.index)) return;

src/linkgraph/linkgraphjob.h

@@ -279,6 +279,7 @@ public:
 	~LinkGraphJob();
 
 	void Init();
+	void FinaliseJob();
 
 	bool IsJobCompleted() const;
 

src/linkgraph/linkgraphschedule.cpp

@@ -129,7 +129,8 @@ void LinkGraphSchedule::JoinNext()
 		std::unique_ptr<LinkGraphJob> next = std::move(this->running.front());
 		this->running.pop_front();
 		LinkGraphID id = next->LinkGraphIndex();
-		next.reset(); // implicitly joins the thread
+		next->FinaliseJob(); // joins the thread and finalises the job
+		next.reset();
 		if (LinkGraph::IsValidID(id)) {
 			LinkGraph *lg = LinkGraph::Get(id);
 			this->Unqueue(lg); // Unqueue to avoid double-queueing recycled IDs.