Initialize const void *idata to NULL.

This commit makes the main leo() loop initialize the idata pointer
(passed by pointer to in()) to NULL. This fixes the following two
potential bugs:
1. If the first in() call did not set leo()'s idata, it would attempt to
use an invalid (random) pointer for data.
2. in() may rely on idata initially being set to NULL, e.g. in the case
of realloc().