builtins: spawn: set `out`/`err` to NULL in child_cleanup
builtins: spawn: set `out`/`err` to NULL in child_cleanup

So they don't get double freed in child_destroy.

Signed-off-by: Yuxuan Shui <7f7e1535bcd10fa942d56a3436533af20f33203f@gmail.com>

yshui/deai,yshui/deai,yshui/deai