lua-factory: don't iterate on lua env table
lua-factory: don't iterate on lua env table

When calling a c function in lua, the top of the stack is the
environment table of lua, _G.

This patch fixes the case where GrlNetWc options was not passed
at grl.fetch and grl.unzip functions.

https://bugzilla.gnome.org/show_bug.cgi?id=733289

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