NTP: ensure parser name is not freed after registration

[chifflier]

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• I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
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• I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)
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Link to redmine ticket:
2414

Describe changes:

• The parser name allocated in rs_register_ntp_parser is passed to the C layer to register the parser, which stores the pointer.
  However, if a CString is allocated for that, it is freed at function exit.
• Ensure parser name is not freed after registration

[victorjulien]

Merged in #3157, thanks!