nlp/lg-parse: fix Clang warning in LGParseLink::execute()
nlp/lg-parse: fix Clang warning in LGParseLink::execute()

strncat() is used here in a wrong way which is seen by Clang static-analysis
as "Size argument is greater than the free space in the destination
buffer". Actually with given sizes of buffers and their contents it's not a
problem, but make code a little bit better here anyway.

andre-senna/opencog,AmeBel/opencog,AmeBel/opencog,AmeBel/opencog,AmeBel/opencog,AmeBel/opencog,andre-senna/opencog,andre-senna/opencog,AmeBel/opencog,andre-senna/opencog,andre-senna/opencog,andre-senna/opencog,AmeBel/opencog,andre-senna/opencog