[LyX/master] Prevent division by zero

[Richard Kimberly Heck]

On 6/15/24 08:52, Jürgen Spitzmüller wrote:
> Am Samstag, dem 15.06.2024 um 12:50 +0000 schrieb Juergen Spitzmueller:
>> commit 0fc8ee5ace22c11175cb93f5b6f7310cc4d646c1
>> Author: Juergen Spitzmueller <spitz at lyx.org>
>> Date:   Sat Jun 15 14:48:37 2024 +0200
>>
>>      Prevent division by zero
>>      
>>      Fixes crash reported at
>>      https://marc.info/?l=lyx-users&m=171842978728676
> Candidate for 2.4.1.

Cherry picked, thanks.

Riki