copy_and_spawn_sgid: Avoid double calls to close()

If close() on infd and outfd succeeded, reset the fd numbers so that
we don't attempt to close them again.

Reviewed-by: Arjun Shankar <arjun@redhat.com>

support/support_capture_subprocess.c

@@ -170,6 +170,7 @@ copy_and_spawn_sgid (char *child_id, gid_t gid)
      support_subprogram because we only want the program exit status, not the
      contents.  */
   ret = 0;
+  infd = outfd = -1;
 
   char * const args[] = {execname, child_id, NULL};