string_list_append: always set util pointer to NULL

string_list_append: always set util pointer to NULL

It is not immediately obvious that the util field may
contain random bytes after appending an item. Especially
since the string_list_insert* functions _do_ explicitly zero
the util pointer.

This does not appear to be a bug in any current git code, as
all callers either fill in the util field immediately or
never use it. However, it is worth it to be less surprising
to new users of the string-list API who may expect it to be
intialized to NULL.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>