Commenting with xid param throws bad_pid error

[lpalazzi]

Expected behavior:
Submitting a comment using xid param should submit correctly

Actual behavior:
Throws polis_err_post_comment_bad_pid error

To Reproduce:

1. Run up-to-date build from edge branch
2. Create a conversation and embed it on a page using data-xid param
3. Submit a comment on the embedded conversation

or

1. Run up-to-date build from edge branch
2. Create a conversation
3. Send a POST request to /api/v3/comments with the following body:

{ "txt": "Comment text.", "pid": "mypid", "conversation_id": <conversation-id>, "vote": -1, "agid": 1, "xid": "any-xid" }

Additional context:

I dug into the handle_POST_comments() code and I think what's happening is an xid is being created without an equivalent pid before the function checks if it needs to create a pid. I managed to fix the issue with these changes:

lpalazzi@5dea271

Not sure if these changes break any intended functionality but it fixes the issue for me.