safer GWS_PROB, avoid log(0)


myisam/ft_nlq_search.c:
  some DBUG_ENTER/RETURN macro added
myisam/ft_parser.c:
  some DBUG_ENTER/RETURN macro added
myisam/ftdefs.h:
  safer GWS_PROB, avoid log(0).
sql/item_func.cc:
  some DBUG_ENTER/RETURN macro added