p2p: ensure inv is GenMsgTx before ToGenTxid in inv processing

and otherwise log that an unknown INV type was received.

In INV processing, when handling transaction type inv messages,
ToGenTxid() expects that we constructed the CInv ourselves or
that we verified that it is for a transaction type CInv.

Therefore, change this `else` branch into an `else if (inv.GenMsgTx())`
to make this safer and log any INVs that fall through.

Author: jnewbery

src/net_processing.cpp

@@ -2672,7 +2672,7 @@ void PeerLogicValidation::ProcessMessage(CNode& pfrom, const std::string& msg_ty
                     // then fetch the blocks we need to catch up.
                     best_block = &inv.hash;
                 }
-            } else {
+            } else if (inv.IsGenTxMsg()) {
                 const GenTxid gtxid = ToGenTxid(inv);
                 const bool fAlreadyHave = AlreadyHaveTx(gtxid, mempool);
                 LogPrint(BCLog::NET, "got inv: %s  %s peer=%d\n", inv.ToString(), fAlreadyHave ? "have" : "new", pfrom.GetId());
@@ -2685,6 +2685,8 @@ void PeerLogicValidation::ProcessMessage(CNode& pfrom, const std::string& msg_ty
                 } else if (!fAlreadyHave && !m_chainman.ActiveChainstate().IsInitialBlockDownload()) {
                     RequestTx(State(pfrom.GetId()), gtxid, current_time);
                 }
+            } else {
+                LogPrint(BCLog::NET, "Unknown inv type \"%s\" received from peer=%d\n", inv.ToString(), pfrom.GetId());
             }
         }