emacs-jabber-0.7.1 passes the buffer name (that is only a string,
has not been created yet) as the second argument to `open-tls-stream'.
The reason the former `open-tls-stream' worked is that it directly
passed the buffer name to `start-process', that creates the buffer
even if it is only a string.  I believe the present one should be
fixed because it is mentioned in the doc-string:

BUFFER is the buffer (or buffer name) to associate with the process.
                     ^^^^^^^^^^^^^^^^
I'll do it later...