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>

.github/workflows/main.yml

@@ -14,8 +14,8 @@ jobs:
     strategy:
       matrix:
         valgrind:
-           - { configure: ''                  , make: 'check' }
-           - { configure: '--enable-valgrind' , make: 'check-valgrind'  }
+           - { configure: ''                  , cflags: '',    make: 'check' }
+           - { configure: '--enable-valgrind' , cflags: '-O2', make: 'check-valgrind'  }
         options:
            - { configure: ''                    }
            - { configure: '--without-libxml2'   }
@@ -35,7 +35,7 @@ jobs:
       - name: Build the library
         run: |
           ./bootstrap.sh
-          ./configure ${{ matrix.options.configure }} ${{ matrix.valgrind.configure }} CFLAGS="-Werror -g3"
+          ./configure ${{ matrix.options.configure }} ${{ matrix.valgrind.configure }} CFLAGS="-Werror -g3 ${{ matrix.valgrind.cflags }}"
           make -j$(nproc)
       - name: Run tests
         run: |
@@ -135,6 +135,11 @@ jobs:
         if: ${{ !failure() }}
         run: |
           cat testbuild.log
+      - name: Error logs
+        if: ${{ failure() }}
+        run: |
+          cat testbuild.log || true
+          cat testerr.log || true
 
   code-style:
     runs-on: ubuntu-20.04