libtracker-sparql: Assert that values are bound in accessors
I saw CRITICAL errors like this occuring when calling tracker_sparql_cursor_get_double():
(org.gnome.Photos:17222): GLib-CRITICAL **: 13:13:55.663: g_ascii_strtod: assertion 'nptr != NULL' failed
This doesn't give a good idea of the cause. Now the same code produces this error:
(org.gnome.Photos:18736): Tracker-CRITICAL **: 13:29:19.328: tracker_sparql_cursor_real_get_double: assertion 'tracker_sparql_cursor_real_is_bound (cursor, column)' failed