Several fixes for bt_iter_pos related functions

Several fixes for bt_iter_pos related functions

Add verification so we free the array only if iterator is of type BT_SEEK_RESTORE
Add return statement at the end of case
Set type of bt_iter_pos returned by bt_iter_get_pos

[ Mathieu Desnoyers edit: check the iter_pos->type before the restore
  pointer in bt_iter_free_pos. The && operator is a sequence point, so
  the first check is done before the second. Since we might not be
  using the union for restore if type != BT_SEEK_RESTORE, check the
  type first before accessing the (possibly wrong) union field. ]

Signed-off-by: Francis Deslauriers <francis.deslauriers@polymtl.ca>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>