Enable strict multisig.

[dajohi]

The reference implementation has a script verification flag SCRIPT_VERIFY_NULLDUMMY, which is used to verify the dummy stack consumed by CHECKMULTISIG is of zero-length. This flag is part of their STANDARD_SCRIPT_VERIFY_FLAGS.

See bitcoin/bitcoin@6380180

This diff should get the job done, but perhaps a flags option would be better since mempool and mining need to stay in sync.

diff --git a/mempool.go b/mempool.go
index d41f946..572a61f 100644
--- a/mempool.go
+++ b/mempool.go
@@ -905,7 +905,8 @@ func (mp *txMemPool) maybeAcceptTransaction(tx *btcutil.Tx, isOrphan *bool, isNe

        // Verify crypto signatures for each input and reject the transaction if
        // any don't verify.
-       flags := btcscript.ScriptBip16 | btcscript.ScriptCanonicalSignatures
+       flags := btcscript.ScriptBip16 | btcscript.ScriptCanonicalSignatures |
+               btcscript.ScriptStrictMultiSig
        err = btcchain.ValidateTransactionScripts(tx, txStore, flags)
        if err != nil {
                return err
diff --git a/mining.go b/mining.go
index abeb952..4bc3303 100644
--- a/mining.go
+++ b/mining.go
@@ -669,7 +669,8 @@ mempoolLoop:
                        logSkippedDeps(tx, deps)
                        continue
                }
-               flags := btcscript.ScriptBip16 | btcscript.ScriptCanonicalSignatures
+               flags := btcscript.ScriptBip16 | btcscript.ScriptCanonicalSignatures |
+                       btcscript.ScriptStrictMultiSig
                err = btcchain.ValidateTransactionScripts(tx, blockTxStore, flags)
                if err != nil {
                        minrLog.Tracef("Skipping tx %s due to error in "+