Fix segfault when shutting down before fully loading

This was introduced by 3192975.
It can be triggered easily when canceling DB upgrade from
pre-per-utxo.

src/init.cpp

@@ -263,7 +263,9 @@ void PrepareShutdown()
     }
 
     // FlushStateToDisk generates a SetBestChain callback, which we should avoid missing
-    FlushStateToDisk();
+    if (pcoinsTip != nullptr) {
+        FlushStateToDisk();
+    }
 
     // After there are no more peers/RPC left to give us new data which may generate
     // CValidationInterface callbacks, flush them...