Plug a memory leak.

Addressing:
 7 bytes in 1 blocks are definitely lost in loss record 1 of 2
    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x33B44AE548: xmlStrndup (xmlstring.c:45)
    by 0x4C6A11B: ds_rds_add_ai_from_xccdf_results (rds.c:332)
    by 0x4C6AF4E: ds_rds_add_xccdf_test_results (rds.c:709)
    by 0x4C6B333: ds_rds_create_from_dom (rds.c:781)
    by 0x4C6B5F9: ds_rds_create (rds.c:847)
    by 0x4CFE086: xccdf_session_export_arf (xccdf_session.c:1340)
    by 0x40BF41: app_evaluate_xccdf (oscap-xccdf.c:515)
    by 0x407EA0: oscap_module_call (oscap-tool.c:261)
    by 0x40831D: oscap_module_process (oscap-tool.c:346)
    by 0x406CD6: main (oscap.c:79)