Fix replacing a regular file with a dir for ARCHIVE_EXTRACT_SAFE_WRITES

Fix replacing a regular file with a dir for ARCHIVE_EXTRACT_SAFE_WRITES

The outer if checks !S_ISDIR(a->st.st_mode), so we know that the file
being overwritten is not a directory, and thus we can rename(2) over it
if we want to, but whether we can use a temporary regular file is a
property of the file being extracted. Otherwise, when replacing a
regular file with a directory, we end up in this case and create a
temporary regular file for the new directory, but with the permissions
of the directory (which likely includes x), and rename it over the top
at the end. Depending on where the archive_entry came from, it may have
a non-zero size that also isn't ovewritten with 0 (e.g. if it came from
stat(2)) and so the API user may then try to copy data (thus failing if
read(2) of directories isn't permitted, or writing the raw directory
contents if it is), but if the size is zero as is the case for this tar
test then it will end up not writing any data and "successfully"
overwrite the file with an empty file, not a directory.