TxBuilder: fix undefined scriptSig

Fixed and typeForce used to enforce this wont happen again in future.

src/transaction.js

@@ -308,6 +308,9 @@ Transaction.prototype.toHex = function() {
 }
 
 Transaction.prototype.setInputScript = function(index, script) {
+  typeForce('Number', index)
+  typeForce('Script', script)
+
   this.ins[index].script = script
 }
 

src/transaction_builder.js

@@ -249,12 +249,15 @@ TransactionBuilder.prototype.__build = function(allowIncomplete) {
       }
     }
 
-    // if we built a scriptSig, wrap as scriptHash if necessary
-    if (scriptSig && input.prevOutType === 'scripthash') {
+    // did we build a scriptSig?
+    if (scriptSig) {
+      // wrap as scriptHash if necessary
+      if (input.prevOutType === 'scripthash') {
         scriptSig = scripts.scriptHashInput(scriptSig, input.redeemScript)
       }
 
       tx.setInputScript(index, scriptSig)
+    }
   })
 
   return tx