qtest: Assure that init_socket()'s listen() does not fail

In practice this seems very unlikely, so cleanup is neglected, as done
for bind().

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/tests/libqtest.c b/tests/libqtest.c
index 8155695..232f781 100644
--- a/tests/libqtest.c
+++ b/tests/libqtest.c
@@ -72,7 +72,8 @@
         ret = bind(sock, (struct sockaddr *)&addr, sizeof(addr));
     } while (ret == -1 && errno == EINTR);
     g_assert_no_errno(ret);
-    listen(sock, 1);
+    ret = listen(sock, 1);
+    g_assert_no_errno(ret);
 
     return sock;
 }