gspawn, win32: fix redirecting only stderr

Problem was just a typo - wrong variable was checked before enabling
stderr redirection.
This fixes error-only redirection spawn-test added in previous commit.

Behavior while redirecting only stdout should be unaffected,
since old code tried to redirect stderr to -1 in such case,
which silently failed I think.

glib/gspawn-win32.c

@@ -657,7 +657,7 @@ do_spawn_with_fds (gint                 *exit_status,
       new_argv[ARG_STDOUT] = "-";
     }
   
-  if (stdout_fd != -1)
+  if (stderr_fd != -1)
     {
       _g_sprintf (args[ARG_STDERR], "%d", stderr_fd);
       new_argv[ARG_STDERR] = args[ARG_STDERR];