Pull in fix from upstream NetBSD rev. 1.5:

If Unwind_Backtrace is broken, ctx.n will still contain ~0, and we will
return that which poor behavior for the user, so return 0 instead.
We c

Pull in fix from upstream NetBSD rev. 1.5:

If Unwind_Backtrace is broken, ctx.n will still contain ~0, and we will
return that which poor behavior for the user, so return 0 instead.
We could document ~0 to be an error, but that would deviate from the
Linux behavior which is not desirable. Noted by Poul-Henning Kamp

PR: 209842

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