Avoid exploding when magic_words are not counted

See #18.

etherpump/commands/pull.py

@@ -529,11 +529,10 @@ async def handle_pad(args, padid, data, info, session):
         async with await trio.open_file(metapath, "w") as f:
             await f.write(json.dumps(meta))
 
-    mwords_msg = (
-        ", magic words: {}".format(", ".join(magic_words))
-        if magic_words
-        else ""
-    )
+    try:
+        mwords_msg = ", magic words: {}".format(", ".join(magic_words))
+    except UnboundLocalError:
+        mwords_msg = ""  # Note(decentral1se): for when magic_words are not counted
 
     print("[x] {} (saved{})".format(padid, mwords_msg))
     saved += 1