don't make a copy of m_non_base_coins

Author: glozow

src/txmempool.h

@@ -940,7 +940,7 @@ class CCoinsViewMemPool : public CCoinsViewBacked
      * m_temp_added and cannot be flushed to the back end. Only used for package validation. */
     void PackageAddTransaction(const CTransactionRef& tx);
     /** Get all coins in m_non_base_coins. */
-    std::unordered_set<COutPoint, SaltedOutpointHasher> GetNonBaseCoins() const { return m_non_base_coins; }
+    const std::unordered_set<COutPoint, SaltedOutpointHasher>& GetNonBaseCoins() const { return m_non_base_coins; }
     /** Clear m_temp_added and m_non_base_coins. */
     void Reset();
 };