In mixed core notes, don't let handle_core_item repeat.

In mixed core notes, don't let handle_core_item repeat.

If a core note contains both registers and items, descsz is 0 to express
that we don't wish to repeat the items.  If there is only one item in
such note, a special block of code hits that passes &size to
handle_core_item, which will decrease that size by the amount consumed by
the item.  But because size is 0, it underflows and wraps, and the loop
following this block, which handles the common case, overruns the core
note buffer.

Signed-off-by: Petr Machata <pmachata@redhat.com>