bug#1295: Segfault with `M-- M-r' when `scroll-margin' > 0

[martin rudalics]

>> Emacs crashes with segmentation fault when I type command `M-- M-r'
>> while variable `scroll-margin' has value 1 more more.
>>
>>  1. $ emacs -Q
>>
>>  2. C-h r
>>
>>  3. Don't move the cursor. Set the `scroll-margin' variable:
>>
>>         M-x set-variable RET scroll-margin RET 4 RET
>>
>>  4. M-- M-r
>>
>>         Fatal error (11)Segmentation fault
>
> Hi Martin,
>
> I think this may be a consequence of your 2008-11-02 checkin to src (I
> didn't pinpoint the exact change that caused it, though).
>
> Could you check?

The report was for Emacsen built as

    GNU Emacs 22.2.1 (i486-pc-linux-gnu, GTK+ Version 2.12.11) of
    2008-10-17 on raven, modified by Debian

and development snapshot version

    GNU Emacs 23.0.60.1 (i486-pc-linux-gnu, GTK+ Version 2.12.11) of
    2008-10-18 on elegiac, modified by Debian

and I verified here that it crashes with a negative argument for all
versions I keep personally.

BTW, Richard apparently once tried to make `move-to-window-line' handle
scroll margins but failed somehow.

martin