Fix: error path if heap_init fails

Avoid an eventual double-free if heap_init fails in bt_iter_set_pos.

[ Note by Mathieu Desnoyers: the current implementation of heap_init
  ensures that ptrs is set to NULL when it fails, so there is currently
  no double-free situation. But let's not rely on this implementation
  detail that might change in the future. ]

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>