youtube: Fix default values of GDataYouTubeQuery location properties

The latitude and longitude properties previously had 0.0 as their
default value, which is a valid location. They should instead have
G_MAXDOUBLE as their default, which is not a valid location — and hence
doesn’t affect query results before the properties are set explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=687597