Fix "invalid URL" while trying to download from Libgen.lc due to relative links are returned from the server.

[petruchito]

This fixes the following problem:

[11:46:48 PM] Added to the download queue.
[11:46:48 PM] Started.
[11:46:48 PM] Downloading page: http://libgen.lc/scimag/ads.php?doi=10.1093/ajcn/65.3.737
[11:46:48 PM] Request:
GET http://libgen.lc/scimag/ads.php?doi=10.1093/ajcn/65.3.737
User-Agent: LibgenDesktop/1.4.1
[11:46:49 PM] Server response:
200 OK
Connection: keep-alive
Vary: Accept-Encoding, Accept-Encoding
Date: Tue, 24 Aug 2021 20:46:48 GMT
Server: nginx
Content-Length: 3273
Content-Type: text/html
[11:46:49 PM] Transformation libgen_lc_scimag returned an invalid URL.