Check against MANDATORY flags prior to accepting to mempool

Previously transactions were only tested again the
STANDARD_SCRIPT_VERIFY_FLAGS prior to mempool acceptance, so any bugs in
those flags that allowed actually-invalid transactions to pass would
result in allowing invalid transactions into the mempool. Fortunately
there is a second check in CreateNewBlock() that would prevent those
transactions from being mined, resulting in an invalid block, however
this could still be exploited as a DoS attack.

src/main.cpp

@@ -1039,6 +1039,21 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         {
             return error("AcceptToMemoryPool: : ConnectInputs failed %s", hash.ToString());
         }
+
+        // Check again against just the consensus-critical mandatory script
+        // verification flags, in case of bugs in the standard flags that cause
+        // transactions to pass as valid when they're actually invalid. For
+        // instance the STRICTENC flag was incorrectly allowing certain
+        // CHECKSIG NOT scripts to pass, even though they were invalid.
+        //
+        // There is a similar check in CreateNewBlock() to prevent creating
+        // invalid blocks, however allowing such transactions into the mempool
+        // can be exploited as a DoS attack.
+        if (!CheckInputs(tx, state, view, true, MANDATORY_SCRIPT_VERIFY_FLAGS, true))
+        {
+            return error("AcceptToMemoryPool: : BUG! PLEASE REPORT THIS! ConnectInputs failed against MANDATORY but not STANDARD flags %s", hash.ToString());
+        }
+
         // Store transaction in memory
         pool.addUnchecked(hash, entry);
     }