macOS NS GUI crash due to invalid font pointer in frame struct while marking

[Daniel Pittman <slippycheeze@google.com>]

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G'day.  I've come across a crash during GC on macOS 10.15.2, GNU Emacs
built from source, git d97a77c481ec913d8c3c24f2eecdc41a28243678.

The crash is located at:
...->mark_window->mark_vectorlike->mark_frame

In this the bad pointer is in (struct frame).output_data.ns.font, where the
pointer is wildly out of the memory map, and so crashes trying to determine
if that was marked.

I haven't yet tracked this down to a root cause, but before I invest
significant time in that I'd like to know if anyone else is investigating
this, or something similar to this?

Reproduction is, painfully, just a matter of waiting for a crash.  It seems
to be vaguely correlated to external process interactions, but can't find a
clear root cause.

I'll work to get a fix, of course, but wanted to avoid duplicating work if
someone else already has this in hand, or knows more.

Thanks!

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