Swap relay pool and mempool lookup

This is in preparation to using the mempool entering time as part of
the decision for relay, but does not change behavior on itself.

Author: sipa

src/net_processing.cpp

@@ -1627,13 +1627,6 @@ CTransactionRef static FindTxForGetData(CNode& peer, const uint256& txid, const
         if (peer.m_tx_relay->setInventoryTxToSend.count(txid)) return {};
     }
 
-    {
-        LOCK(cs_main);
-        // Look up transaction in relay pool
-        auto mi = mapRelay.find(txid);
-        if (mi != mapRelay.end()) return mi->second;
-    }
-
     auto txinfo = mempool.info(txid);
     if (txinfo.tx) {
         // To protect privacy, do not answer getdata using the mempool when
@@ -1644,6 +1637,13 @@ CTransactionRef static FindTxForGetData(CNode& peer, const uint256& txid, const
         }
     }
 
+    {
+        LOCK(cs_main);
+        // Look up transaction in relay pool
+        auto mi = mapRelay.find(txid);
+        if (mi != mapRelay.end()) return mi->second;
+    }
+
     return {};
 }