Small memory leak in src ACL parse

Small memory leak in src ACL parse

Fixes one minor memory leak when IPv6 is disabled and parsing an IPv6
address. For example the default localhost ACL ::1 value.

Caught by Valgrind:

==26647== 384 bytes in 4 blocks are definitely lost in loss record 1,132 of 1,726
==26647==    at 0x4C25A28: calloc (vg_replace_malloc.c:467)
==26647==    by 0x65B441: xcalloc (xalloc.cc:75)
==26647==    by 0x657B99: MemPoolMalloc::allocate() (MemPoolMalloc.cc:62)
==26647==    by 0x5A95B1: acl_ip_data::FactoryParse(char const*) (Ip.h:66)
==26647==    by 0x5AA8BD: ACLIP::parse() (Ip.cc:523)
==26647==    by 0x5E0A80: ACL::ParseAclLine(ConfigParser&, ACL**) (Acl.cc:174)
==26647==    by 0x4B0C0F: parse_line(char*) (cache_cf.cc:1252)
==26647==    by 0x4B2076: parseOneConfigFile(char const*, unsigned int) (cache_cf.cc:518)
==26647==    by 0x4B29D0: parseConfigFile(char const*) (cache_cf.cc:558)
==26647==    by 0x546B81: SquidMain(int, char**) (main.cc:1372)
==26647==    by 0x547445: main (main.cc:1215)