[tlbuild] Repeated Mac arm64 crashes of xdvi-xaw with "set_no_char: attempt to set character of unknown font, offset 42"

[Karl Berry]

    build with the various clang "sanitizer"

Maybe -fsanitize=address (CFLAGS and LDFLAGS) would show something.

Although I suspect if such a memory error was in the xdvi code, it would
show on more platforms.
    
    Does somebody running x86_64 linux (or anything, really) want to
    give valgrind a shot on xdvi with t2a.dvi file? It's quite plausible
    that valgrind will catch a problem on an architecture that doesn't
    happen to crash.

Unfortunately, it didn't. On my CentOS 7 x86_64-linux, I ran valgrind
(3.15.0, what's available in the distros) on the 2021 xdvi-xaw binary:

  valgrind xdvi-xaw t2a.dvi 

I hit R a bunch of times, scrolled around some, hit R some more, etc.
No meaningful problems reported. Drat.

Since Johannes couldn't reproduce on FreeBSD either, I surmise it maybe
is somehow specific to Macs.

Maybe per Luigi, set a watchpoint on m_dvi_fp (or whatever the variable
name is) and see when it changes to zero? Maybe the reopening of the dvi
file on R is failing? -k