wait for mining threads to fully exit before stopping

src/miner.cpp

@@ -1986,6 +1986,8 @@ void static BitcoinMiner()
         if (minerThreads != NULL)
         {
             minerThreads->interrupt_all();
+            // Wait for all miner threads to fully terminate before destroying shared resources
+            minerThreads->join_all();
             delete minerThreads;
             minerThreads = NULL;