Fix stack overflow in destructor when dnsbl object is kept too long alive

`shared_ptr`s of `CServer::m_pDnsblLookup[ClientID]` are set, but not
cleaned up when done. Therefore the Job is kept alive until the player
disconnects and a new player joins on that slot. Currently this means
that the full linked list of jobs is kept alive.

When the Job is overwritten with a new job, all the remaining objects in
the list can be dropped. With enough jobs, that is causing a stack
overflow in the destructor.

This patch fixes this overflow by making the lifetime independent of the
previous Job. Jobs can get dropped after processing them, even when the
previous job is still alive.

Fixes #6954

src/engine/shared/jobs.cpp

@@ -62,6 +62,8 @@ void CJobPool::WorkerThread(void *pUser)
 			{
 				pJob = pPool->m_pFirstJob;
 				pPool->m_pFirstJob = pPool->m_pFirstJob->m_pNext;
+				// allow remaining objects in list to destruct, even when current object stays alive
+				pJob->m_pNext = nullptr;
 				if(!pPool->m_pFirstJob)
 					pPool->m_pLastJob = 0;
 			}