bug#48422: Cannot print backtrae from system-async-mark

Severity: minor

Hi guilers,

I have some trouble making backtraces with system-async-mark.
(Not encountered in ‘real code’, only when playing with the REPL.)

From a REPL:

scheme@(guile-user)> (system-async-mark backtrace )

Backtrace:
Exception thrown while printing backtrace:
In procedure program-code: Wrong type argument in position 1 (expecting PROGRAM_P): #<unspecified>

Greetings,
Maxime.

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