tar: Reset accumulated header state after reading macOS metadata blob

tar: Reset accumulated header state after reading macOS metadata blob

AppleDouble extension entries are present as separate files immediately
preceding the corresponding real files. In libarchive, we process the
entire metadata file (headers + data) as if it were a header in the real
file. However, the code forgets to reset the accumulated header state
before parsing the real file's headers. In one code path, this causes
the metadata file's name to be used as the real file's name.

Specifically, this can be triggered with a tar containing two files:

1. A file named `._badname` with pax header containing the `path` attribute
2. A file named `goodname` _with_ a pax header but _without_ the `path` attribute

libarchive will list one file, `._badname` containing the data of `goodname`.

This code is pretty brittle and we really should let the client deal with
it :(

Fixes #2510.

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>