bug#20774: auto-fill doesn't work properly when first-line prefix differs in adaptive-fill-mode

[npostavs]

Samuel Freilich <sfreilich@google.com> writes:

> $ emacs -Q -l markdown-mode.el
> M-x markdown-mode RET
> M-x auto-fill-mode RET
> M-x set-fill-column RET 5 RET
>
> Type the following into the buffer and hit RET:
> * Item
>
> Actual result:
> *
>   Item
>   [cursor]
>
> Expected result:
> * Item
>   [cursor]

With the patch I get

* Item
[cursor]

i.e, the cursor lands in column 0.  Is it correct?