gvdb: Fix error handling in gvdb_table_new()

The documentation was unclear about what error codes would be returned
on attempting to open an empty or corrupt GVDB file. Previous versions
of the documentation incorrectly said that corrupt GVDB files were
considered equivalent to empty ones.

A recent commit has clarified the documentation to include its error
handling behaviour.

Update the two users of GVDB within GLib, GResource and GSettingsSource,
to follow this change, and add unit tests for them both.

Other users of the GVDB copylib will need to update their copy and make
appropriate changes if they have bugs in their handling of this
situation. dconf is one example of this. GVDB should be updated from
https://gitlab.gnome.org/GNOME/gvdb.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1454

gio/gsettingsschema.c

@@ -267,6 +267,9 @@ g_settings_schema_source_unref (GSettingsSchemaSource *source)
  * Generally, you should set @trusted to %TRUE for files installed by the
  * system and to %FALSE for files in the home directory.
  *
+ * In either case, an empty file or some types of corruption in the file will
+ * result in %G_FILE_ERROR_INVAL being returned.
+ *
  * If @parent is non-%NULL then there are two effects.
  *
  * First, if g_settings_schema_source_lookup() is called with the