CC Mode: Fix positioning of point whilst inserting comments without non-ws

CC Mode: Fix positioning of point whilst inserting comments without non-ws

* lisp/progmodes/cc-cmds.el (c-guess-fill-prefix): When determining a new
block comment prefix (i.e. there isn't one already there to copy), and that
prefix is hard up against a comment closer, ensure there are at least two
spaces before the closer.
(c-indent-new-comment-line): Amend the strategy for ensuring that point isn't
left hard up against the comment closer after M-j.