podcasts: Fix crasher when DB could not be opened
podcasts: Fix crasher when DB could not be opened

Don't try to close the database if it is invalid.

https://bugzilla.gnome.org/show_bug.cgi?id=741258

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