trac#480: plug a memory leak
trac#480: plug a memory leak

Addressing:
   at 0x4C2B946: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E7F818: __oscap_calloc (alloc.c:68)
   by 0x4E80DC2: oscap_iterator_new (list.c:242)
   by 0x4EE1D2D: _xccdf_text_substitution_cb (xccdf_policy_substitute.c:174)
   by 0x4E84727: xml_element_dfs_callback (xml_iterate.c:31)
   by 0x4E84775: xml_element_dfs_callback (xml_iterate.c:36)
   by 0x4E8481E: xml_iterate_dfs (xml_iterate.c:70)
   by 0x4EE2124: xccdf_policy_resolve_fix_substitution (xccdf_policy_substitute.c:201)
   by 0x4EE1528: xccdf_policy_rule_result_remediate (xccdf_policy_remediate.c:387)
   by 0x4EE1B44: xccdf_policy_remediate (xccdf_policy_remediate.c:439)
   by 0x40B7B9: app_evaluate_xccdf (oscap-xccdf.c:513)
   by 0x407BFC: oscap_module_call (oscap-tool.c:260)
   by 0x407BFC: oscap_module_process (oscap-tool.c:345)

OpenSCAP/openscap,mpreisler/openscap,jan-cerny/openscap,postfix/openscap,OpenSCAP/openscap,redhatrises/openscap,mpreisler/openscap,redhatrises/openscap,mpreisler/openscap,jan-cerny/openscap,openprivacy/openscap,Hexadorsimal/openscap,OpenSCAP/openscap,OpenSCAP/openscap,jan-cerny/openscap,mpreisler/openscap,Hexadorsimal/openscap,Hexadorsimal/openscap,ybznek/openscap,jan-cerny/openscap,OpenSCAP/openscap,openprivacy/openscap,mpreisler/openscap,jan-cerny/openscap,ybznek/openscap,ybznek/openscap,postfix/openscap,redhatrises/openscap,Hexadorsimal/openscap,ybznek/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,Hexadorsimal/openscap,postfix/openscap,openprivacy/openscap,openprivacy/openscap,postfix/openscap,openprivacy/openscap,postfix/openscap,Hexadorsimal/openscap,ybznek/openscap,postfix/openscap,redhatrises/openscap,ybznek/openscap,mpreisler/openscap,redhatrises/openscap,openprivacy/openscap