gspawn: close child_err_report_fd before exiting on error

When the child process is going to exit on error after fork() but before
exec(), let's close the child_err_report_fd. The practical value of this
is to placate valgrind --track-fds=yes.

glib/gspawn-posix.c

@@ -547,6 +547,8 @@ write_err_and_exit (gint fd, gint msg)
   write_all (fd, &msg, sizeof(msg));
   write_all (fd, &en, sizeof(en));
 
+  close (fd);
+  
   _exit (1);
 }