fix: handle possible nullptr in InitNewQuorum

Author: knst

src/llmq/dkgsessionhandler.cpp

@@ -539,7 +539,7 @@ void CDKGSessionHandler::HandleDKGRound(CConnman& connman, PeerManager& peerman)
 
     const CBlockIndex* pQuorumBaseBlockIndex = WITH_LOCK(::cs_main, return m_chainstate.m_blockman.LookupBlockIndex(curQuorumHash));
 
-    if (!InitNewQuorum(pQuorumBaseBlockIndex)) {
+    if (!pQuorumBaseBlockIndex || !InitNewQuorum(pQuorumBaseBlockIndex)) {
         // should actually never happen
         WaitForNewQuorum(curQuorumHash);
         throw AbortPhaseException();