Initialize subst rule->result pointer to NULL

Initialize subst rule->result pointer to NULL

This is not directly useful in libarchive at the present time, but
it might be in the future.  If a project which uses this code adds
an atexit(3) handler to clean up memory, and somebody runs:
    bsdtar somestring -t
then "omestring" will be interpreted as an (invalid) substitution
rule and call lafe_errc().  However, if the atexit() handler then
calls cleanup_substition(), it will try to free(rule->result)
which was not initialized.

Sponsored by:	Tarsnap Backup Inc.