Fix segfault when deleting and reusing attachment slots. (closes #3800)

Fix segfault when deleting and reusing attachment slots. (closes #3800)

When attachments are deleted, delete_attachment() slides the entries
down in the idx array, but forgets to NULL out the last vacated slot.

If more attachments are added later on via OP_COMPOSE_EDIT_HEADERS and
the Attach: pseudo-header, mutt_gen_attach_list() will attempt to
re-use the ATTACHPTR in that last slot because it wasn't set to NULL.
This will be pointing to freed memory and likely segfault (at best).