Fix off-by-one error in TarWriter.

The tar format originally supported up to 99 byte filename. The two
extensions are proposed later: Ustar or PAX.

In the UStar extension, a pathanme is split at a '/' and its "prefix"
and "suffix" are stored in different locations in the tar header. Since
"prefix" can be up to 155 byte, it can represent up to 254 byte
filename (but exact limit depends on the location of '/' character in
a pathname.)

Our TarWriter first attempt to use UStar extension and then fallback to
PAX extension.

But there's a bug in UStar header creation. "Suffix" part must be a NUL-
terminated string, but we didn't handle it correctly. As a result, if
your filename just 100 characters long, the last character was droppped.

This patch fixes the issue.

Differential Revision: https://reviews.llvm.org/D38149

llvm-svn: 314349