describe_backtrace() argument shorting sometimes extends the argument.

Imported from https://youtrack.roxen.com/issue/PIKE-224

Reported by @grubba

From Roxen 6.3.333 selftest:

>> ### Thread 0x8 - Handle Thread [4] - busy for 602.854s:
>> pike/lib/master.pike:1838: file_stat("/var/tmp/dist-test/roxen-6.3.333-nb-ep-sol11_x86_64.sh/roxen/var/rep_test_repository/Test_REP.sb/print/db/spool/testdir3/test_x_put_copy_mo"+[-9]+"t_copy_move_delete/Cyrillic-Ñ\204Ñ\211Ñ\212Ð\202Ð\203Ð\204Ð\211Ð\226",UNDEFINED)
>> pike/lib/modules/Stdio.pmod/module.pmod:2830: Stdio.exist("/var/tmp/dist-test/roxen-6.3.333-nb-ep-sol11_x86_64.sh/roxen/var/rep_test_repository/Test_REP.sb/print/db/spool/testdir3/test_x_put_copy_move_delete/Cyrillic-Ñ\204Ñ\211Ñ\212Ð\202Ð\203Ð\204Ð\211Ð\226")

Note the [-9] in the attempt to shorten the argument to file_stat().

Observed in Pike 8.0.888.