StThemeNode: handle url() in inline styles better

StThemeNode: handle url() in inline styles better

non-absolute paths specified as url()'s in
stylesheets are resolved to be relative to the location
of the stylesheets they are in.

Inline styles don't have physical styleshseets sitting on disk,
which leads to a crash in the url resolving code.

This commit ensures that we don't try to use the stylesheet associated
with a url, if there isn't one to use.

This commit doesn't try to handle relative paths in inline styles.
It only prevents crashes when absolute paths are used.

https://bugzilla.gnome.org/show_bug.cgi?id=636975