Fix skipping stanzas in escalus_client:peek_stanzas/1

After merging `#client` and `#transport` records in #25 `peek_stanzas/1` is never matching the first clause inside `fun` because `MConn` is now a record `#client` with `jid = undefined` and `Conn` has this field defined.

It might have been responsible for some false positives while using `escalus_assert:has_no_stanzas/1`.