fix txb.sign having issues when an incomplete TX contains OP_0s, but not enough

src/transaction_builder.js

@@ -384,7 +384,7 @@ TransactionBuilder.prototype.sign = function (index, keyPair, redeemScript, hash
 
       // check for any matching signatures
       unmatched.some(function (signature, i) {
-        if (!keyPair2.verify(signatureHash, signature)) return false
+        if (!signature || !keyPair2.verify(signatureHash, signature)) return false
         match = signature
 
         // remove matched signature from unmatched