youtube: Fix potential crasher
youtube: Fix potential crasher

ytsrc might be uninitialised, so abort early in that case when called
from read_url_async().

https://bugzilla.gnome.org/show_bug.cgi?id=721642

vrutkovs/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,jasuarez/grilo-plugins