fix latent bugs in ui-out.c

The destructor code in ui-out.c has a latent bug, which is hidden by
the fact that nothing uses this right now.  This patch fixes the
problem.  The bug is that we don't always clear a pointer in the
ui-out object, leading to a bad free.

2014-03-17  Tom Tromey  <tromey@redhat.com>

	* ui-out.c (clear_table, ui_out_new): Clear uiout->table.id.