services/facebook: fix incorrect facebook video being downloaded (#1499)

[sunilpar]

fixes #1499

summary

fixes an issue where incorrect facebook video was being downloaded due to regex matching a different url from the fetched html.

cause

in some low resolution facebook videos using the share/:shareType/:id pattern, there is no hd url present.

example:

{
  "browser_native_sd_url": "https://video.fktm3-1.fna.fbcdn.net/o1/v/t2/f2/m366/...",
  "browser_native_hd_url": null,
  "id": "1972277293595444"
}

the previous regex:

const hd = html.match('"browser_native_hd_url":(".*?")');
const sd = html.match('"browser_native_sd_url":(".*?")');

would incorrectly match another video's hd url, resulting in downloading the wrong video.

changes

1. added null checks before processing hd/sd urls

const hd = html.match('"browser_native_hd_url":(null|".*?")');
const sd = html.match('"browser_native_sd_url":(null|".*?")');

2. ensured only valid urls are parsed and used

if (hd?.[1] && hd[1] !== 'null') urls.push(JSON.parse(hd[1]));
if (sd?.[1] && sd[1] !== 'null') urls.push(JSON.parse(sd[1]));

if there is a better approach for filtering null or invalid urls, suggestions are welcome.