Plug a memory leak.

Addressing:
 37 bytes in 1 blocks are definitely lost in loss record 2 of 3
    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x31EAC860B9: strdup (in /usr/lib64/libc-2.18.so)
    by 0x4C5DBDA: oscap_strdup (util.c:65)
    by 0x4CB1AEF: xccdf_session_set_report_export (xccdf_session.c:291)
    by 0x40B9D1: app_evaluate_xccdf (oscap-xccdf.c:507)
    by 0x4079BF: oscap_module_process (oscap-tool.c:261)
    by 0x4069BE: main (oscap.c:79)