Add enum for nullifier type

src/main.cpp

@@ -1270,7 +1270,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     }
     BOOST_FOREACH(const JSDescription &joinsplit, tx.vjoinsplit) {
         BOOST_FOREACH(const uint256 &nf, joinsplit.nullifiers) {
-            if (pool.nullifierExists(nf, false))
+            if (pool.nullifierExists(nf, SPROUT_NULLIFIER))
                 return false;
         }
     }