Verify `makeAttachmentPointer` and `encryptAttachment` arguments

The underlying `crypto.subtle.encrypt` API requires it.

libtextsecure/crypto.js

@@ -87,6 +87,13 @@
         },
 
         encryptAttachment: function(plaintext, keys, iv) {
+            if (!(plaintext instanceof ArrayBuffer) && !ArrayBuffer.isView(plaintext)) {
+              throw new TypeError(
+                '`plaintext` must be an `ArrayBuffer` or `ArrayBufferView`; got: ' +
+                typeof plaintext
+              );
+            }
+
             if (keys.byteLength != 64) {
                 throw new Error("Got invalid length attachment keys");
             }

libtextsecure/sendmessage.js

@@ -116,10 +116,21 @@ function MessageSender(url, username, password, cdn_url) {
 
 MessageSender.prototype = {
     constructor: MessageSender,
+
+//  makeAttachmentPointer :: Attachment -> Promise AttachmentPointerProto
     makeAttachmentPointer: function(attachment) {
         if (typeof attachment !== 'object' || attachment == null) {
             return Promise.resolve(undefined);
         }
+
+        if (!(attachment.data instanceof ArrayBuffer) &&
+            !ArrayBuffer.isView(attachment.data)) {
+          return Promise.reject(new TypeError(
+            '`attachment.data` must be an `ArrayBuffer` or `ArrayBufferView`; got: ' +
+            typeof attachment.data
+          ));
+        }
+
         var proto = new textsecure.protobuf.AttachmentPointer();
         proto.key = libsignal.crypto.getRandomBytes(64);