pdbtool: Initialize module_path

With the reloc patch that went in a while ago, module_path has to be
explicitly initialized, otherwise it will remain empty. And if it
remains empty, and we try to split it, GLib will spew warnings at us.

This patch adds the initialisation.

Reported-by: Andras Mitzki <micek@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>