bitcoin-chainstate: Lock cs_main to UnloadBlockIndex

This was introduced because of a silent merge conflict.

src/bitcoin-chainstate.cpp

@@ -256,7 +256,7 @@ epilogue:
     }
     GetMainSignals().UnregisterBackgroundSignalScheduler();
 
-    UnloadBlockIndex(nullptr, chainman);
+    WITH_LOCK(::cs_main, UnloadBlockIndex(nullptr, chainman));
 
     init::UnsetGlobals();
 }