pax writer: fix multiple security vulnerabilities

pax writer: fix multiple security vulnerabilities

Security vulnerabilities:
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()

The vulnerabilities can be triggered when writing pax archives
with extended attributes (SCHILY or LIBARCHIVE) by feeding attribute
names longer than INT_MAX or attribute names that fail to be encoded
properly.

Reported-by: Bahaa Naamneh of Crosspoint Labs