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
Bug 1098: improve invalid pcre/R handling

When not using a file_data or similar 'sticky buffer', a pcre/R option
needs a content in the same buffer.