bug#40217: 28.0.50; Add check for valid month to date-days-in-month

[Eli Zaretskii]

> From: Alex Branham <alex.branham@gmail.com>
> Date: Tue, 24 Mar 2020 19:39:44 -0400
> 
> Here's a patch that adds a check for whether the month argument is
> valid in the new date-days-in-month function. It currently returns 30
> for a month value of, say, 15.

Thanks, pushed to the master branch (after some minor fixes of the
commit log message).