Avoid memory leak in rmtree() when path cannot be opened

Avoid memory leak in rmtree() when path cannot be opened

An allocation done for the directory names to recurse into for their
deletion was done before OPENDIR(), so, assuming that a failure happens,
this could leak a bit of memory.

Author: Ranier Vilela
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAEudQAoN3-2ZKBALThnEk_q2hu8En5A0WG9O+5siJTQKVZzoWQ@mail.gmail.com