coins: reset `cachedCoinsUsage` in Flush() only after the map is cleared

Prevents the counter from dropping to 0 while cacheCoins still holds objects

src/coins.cpp

@@ -254,8 +254,8 @@ bool CCoinsViewCache::Flush() {
     if (fOk) {
         cacheCoins.clear();
         ReallocateCache();
-    }
         cachedCoinsUsage = 0;
+    }
     return fOk;
 }