fix itdb_free crash
fix itdb_free crash

I've seen a few libgpod crashes in itdb_free() which appear to be due to a
double free of itdb->priv->genius_cuid.  Also I assume the check before the
first g_free() can be removed since g_free does nothing if passed NULL.

neuschaefer/libgpod,neuschaefer/libgpod,hyperair/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,hyperair/libgpod,neuschaefer/libgpod,hyperair/libgpod