Fix type of length returned by gvdb_table_get_names()

It should not be unsigned. The type in the on-disk format is gint32, so
we need to return something at least as wide as that. However, we
should not expose the implementation detail that the on-disk format is
specifically gint32. Use a gsize, since that’s the normal type for array
lengths — but check that we’re not on a platform where (somehow) gsize
is smaller than gint32.

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