On AIX, uv_fs_event_start() didn't always initialize
handle->dir_filename. In this scenario, uv_fs_event_stop()
would free the uninitialized pointer. This commit
initialized handle->dir_filename to NULL in all cases.
Fixes: https://github.com/nodejs/node/issues/13577
PR-URL: https://github.com/libuv/libuv/pull/1400
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>