Plug a memory leak.

Addressing:
30 bytes in 1 blocks are definitely lost in loss record 1 of 1
   at 0x4C2745D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x348A2AE548: xmlStrndup (xmlstring.c:45)
   by 0x348A2EBB8C: xmlTextReaderGetAttribute (xmlreader.c:2400)
   by 0x4EFBBBD: cve_model_parse (cve_priv.c:436)
   by 0x4EFBAE5: cve_model_parse_xml (cve_priv.c:414)
   by 0x4EFAA19: cve_model_import (cve.c:60)
   by 0x40E150: app_cve_find (oscap-cve.c:119)
   by 0x407D20: oscap_module_call (oscap-tool.c:261)
   by 0x40819D: oscap_module_process (oscap-tool.c:346)
   by 0x406B56: main (oscap.c:79)
Do not continue parsing, when unknown element occur.
Refactor: cve parser shall check node type only once.