Merge from 3.2: Issue #12926: Fix a bug in tarfile's link extraction.

On platforms that do not support (symbolic) links, tarfile offers a
work-around and extracts a link in an archive as the regular file the link is
pointing to. On other platforms, this code was accidentally executed even
after the link had been successfully extracted which failed due to the already
existing link.

diff --cc Lib/tarfile.py
Simple merge

diff --cc Misc/NEWS
Simple merge