bug#45541: 28.0.50; Frequent crashes on ARM macOS

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Wed, 30 Dec 2020 15:42:24 +0100
> 
> I'm now able to reproduce this consistently. It happens when a tooltip
> appears and you hover over that tooltip

How is that possible?  Tooltips are supposed to pop down as soon as
you move the mouse.  Are those native NS tooltips, and if so, do they
behave differently in this regard?

> The crash is in the line
>   if (f && FRAME_NS_P (f))
> in nsterm.m in ns_mouse_position, so apparently "f" is not NULL, but
> also not valid.

Maybe, if the tooltip popped down, we need to test whether f is a live
frame?