Plug a memory leak.

Addressing:
 59 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 0x4C593D8: __oscap_alloc (alloc.c:52)
    by 0x4C5DD52: oscap_vsprintf (util.c:170)
    by 0x4C5DE0B: oscap_sprintf (util.c:184)
    by 0x4C5BD00: oscap_acquire_guess_realpath (oscap_acquire.c:244)
    by 0x4CB2541: _xccdf_session_export_oval_result_file (xccdf_session.c:1072)
    by 0x4CB44BC: xccdf_session_export_oval (xccdf_session.c:1161)
    by 0x40B72B: app_evaluate_xccdf (oscap-xccdf.c:501)
    by 0x407A7F: oscap_module_process (oscap-tool.c:260)
    by 0x406B1E: main (oscap.c:80)