libarchive: merge security fix from vendor branch

libarchive: merge security fix from vendor branch

This commit fixes a couple of security vulnerabilities in the PAX writer:
1. Heap overflow in url_encode() in archive_write_set_format_pax.c
2. NULL dereference in archive_write_pax_header_xattrs()
3. Another NULL dereference in archive_write_pax_header_xattrs()
4. NULL dereference in archive_write_pax_header_xattr()

Security:	No known reference yet
Obtained from:	https://github.com/libarchive/libarchive/commit/1b4e0d0f9
MFC after:	3 days