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

contrib/libexecinfo/unwind.c

@@ -67,7 +67,9 @@ backtrace(void **arr, size_t len)
 	ctx.n = (size_t)~0;
 
 	_Unwind_Backtrace(tracer, &ctx);
-	if (ctx.n != (size_t)~0 && ctx.n > 0)
+	if (ctx.n == (size_t)~0)
+		ctx.n = 0;
+	else if (ctx.n > 0)
 		ctx.arr[--ctx.n] = NULL;	/* Skip frame below __start */
 
 	return ctx.n;