Fix SM-serialization testcase

`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>

testbuild.sh

@@ -1,6 +1,7 @@
 #!/bin/sh
 
 logfile="../../testbuild.log"
+errfile="../../testerr.log"
 
 err_out() {
   tail $logfile
@@ -8,6 +9,6 @@ err_out() {
 }
 
 ./bootstrap.sh
-./configure >> $logfile || err_out
-make -j$(( `nproc` * 2 + 1 )) >> $logfile || err_out
-make check >> $logfile || err_out
+./configure >> $logfile 2>> $errfile || err_out
+make -j$(( `nproc` * 2 + 1 )) >> $logfile 2>> $errfile || err_out
+make check >> $logfile 2>> $errfile || err_out