nflog: fix memory leaks

This fixes the following memory leaks:

[src/source-nflog.c:222]: (error) Memory leak: ntv
[src/source-nflog.c:236]: (error) Memory leak: ntv
[src/source-nflog.c:253]: (error) Memory leak: ntv
[src/source-nflog.c:258]: (error) Memory leak: ntv