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: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

tests/libqtest.c

@@ -72,7 +72,8 @@ static int init_socket(const char *socket_path)
         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;
 }