Throw exception on bad args to SyncRequest

// FREEBIE

js/libtextsecure.js

@@ -37718,6 +37718,9 @@ textsecure.MessageSender.prototype = {
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
+        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+            throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
+        }
         this.receiver = receiver;
 
         this.oncontact = this.onContactSyncComplete.bind(this);

libtextsecure/sync_request.js

@@ -8,6 +8,9 @@
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
+        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+            throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
+        }
         this.receiver = receiver;
 
         this.oncontact = this.onContactSyncComplete.bind(this);