youtube.lua: handle relative reference in javascript URL
youtube.lua: handle relative reference in javascript URL

(cherry picked from commit 87fcede4b4839ff99dbc52b71664d28c02eb381e)
Signed-off-by: Pierre Ynard <efc674bf215af8c633f25041b289869f9d3ac058@yahoo.fr>

jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1