bug#17560: 24.4.50; wrong type argument if rmail-delete-after-output set

[Ken Olum]

I'd put

(if (not count) (setq count 1))

in rmail-delete-forward and for consistency make the argument of
rmail-delete-backward optional and add the same code there.

Unfortunately, that is not consistent with rmail-next-message, for
example.  The alternative would be to make the argument not optional and
change all callers to say (rmail-delete-forward 1).  Unfortunately, in
previous versions of emacs this would mean to move backward, so new
calls would be incompatible with the old definition, which I think
argues against this plan.

                                        Ken