bin/55568: tar(1) prints "(null)"

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55568
>Category:       bin
>Synopsis:       tar(1) prints "(null)"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Aug 13 16:15:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD 9.0
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Attempting to extract a tar(1) archive overwriting existing files that
one lacks write permission to causes tar to to print confusing
messages that includes the string "(null)".  This probably means tar
is trying to print a null pointer, which arguably falls in the
category of "undefined behavior".

>How-To-Repeat:

As a non-root user:

cd /usr/tests/bin/tar
tar cf /tmp/test.tar .
tar xf /tmp/test.tar

This prints:

./: Can't restore time
./Atffile: (null)
./t_tar: (null)
tar: Error exit delayed from previous errors.

>Fix: