`xmpp_run_once()` uses `FD_SET()` to determine which sockets to `select()`
on. The socket gets initialized to `INVALID_SOCKET = -1` inside
`xmpp_conn_new()` which leads to a buffer overflow once `FD_SET()` is
called.
This is only exposed when enabling a higher optimization level, which
was only done in the `release-test` CI job.
* Fix this testcase by initializing the socket to a possible value.
* Build the Valgrind CI jobs with `-O2`.
* Make the output of the `release-test` more verbose.
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
e3d734c5ff