fix: refuse to create new clsig if we switched to a different fork while we were signing

Author: UdjinM6

src/llmq/chainlocks.cpp

@@ -519,7 +519,10 @@ MessageProcessingResult CChainLocksHandler::HandleNewRecoveredSig(const llmq::CR
             // already got the same or a better CLSIG through the CLSIG message
             return {};
         }
-
+        if (m_chainstate.m_chain.Tip()->GetAncestor(lastSignedHeight)->GetBlockHash() != lastSignedMsgHash) {
+            // we switched to a different fork while we were signing
+            return {};
+        }
 
         clsig = CChainLockSig(lastSignedHeight, lastSignedMsgHash, recoveredSig.sig.Get());
     }