tar: don't cross struct member boundaries with OLDGNU_MAGIC

tar: don't cross struct member boundaries with OLDGNU_MAGIC

* src/create.c (write_gnu_long_link, start_header): Access
header->buffer + offsetof (struct posix_header, magic), instead of
header->header.magic, when reading or writing the OLDGNU_MAGIC
pattern.  The code violates the C standard without this change,
and GCC warns about this if fortify checking is enabled.  It's not
a bug on traditional (i.e., non-debugging) platforms, but it does
violate the C standard so it should be fixed.  Problem originally
reported by John Emil Karlson in
<http://lists.gnu.org/archive/html/bug-tar/2010-04/msg00023.html>.
* src/list.c (decode_header): Likewise.