cpio: fix read-ahead pointer lifetime issue

cpio: fix read-ahead pointer lifetime issue

Cache the CPIO trailer-name check before consuming the pathname
buffer.

The reader kept a pointer returned by __archive_read_ahead() and used
it after __archive_read_consume(), and after possible symlink body
processing. Read-ahead pointers are views into libarchive input buffers
and should not be relied on after the associated bytes are consumed.

Store the TRAILER!!! decision while the pathname buffer is still valid,
then use the saved boolean after the remaining header handling.