Fix memory leaks upon disconnect

Hi there,

First, thanks for libstrophe. I was in the need for a sleek C XMPP library and it comes in really handy. Please merge the two memleak fixes I attached, so that my router won't crash after a few days :).

(1) the connection did not release its sendqueue in xmpp_conn_release.
(2) the connection did not release its TLS context in xmpp_conn_release.

regards,
Jonas