Do not include comment start chars in ffap string

Do not include comment start chars in ffap string

* lisp/ffap.el (ffap-string-at-point): If the point is in a comment,
ensure that the returned string does not contain the comment start
characters (especially for major modes that have '//' as comment start
characters).  Otherwise, in a major mode like c-mode, with `ido-mode'
enabled and `ido-use-filename-at-point' set to `guess', doing "C-x
C-f" on a "//foo" comment will initiate an attempt to access a path
"//foo" (Bug#24057).

Co-authored-by: Noam Postavsky <npostavs@gmail.com>