Do not invoke anti-DoS system for invalid BIP16 transactions

Doing so would allow an attack on old nodes, which would relay a
standard transaction spending a BIP16 output in an invalid way,
until reaching a new node, which will disconnect their peer.

Reported by makomk on IRC.

src/main.cpp

@@ -1138,7 +1138,14 @@ bool CTransaction::ConnectInputs(MapPrevTx inputs,
             {
                 // Verify signature
                 if (!VerifySignature(txPrev, *this, i, fStrictPayToScriptHash, 0))
+                {
+                    // only during transition phase for P2SH: do not invoke anti-DoS code for
+                    // potentially old clients relaying bad P2SH transactions
+                    if (fStrictPayToScriptHash && VerifySignature(txPrev, *this, i, false, 0))
+                        return error("ConnectInputs() : %s P2SH VerifySignature failed", GetHash().ToString().substr(0,10).c_str());
+
                     return DoS(100,error("ConnectInputs() : %s VerifySignature failed", GetHash().ToString().substr(0,10).c_str()));
+                }
             }
 
             // Mark outpoints as spent