file.c: realpath on special symlink

	* file.c (realpath_rec): fallback to symlink path when it is
	  accessible but the link target is not actual entry on file
	  systems.  [ruby-dev:50487] [Bug #14557]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62903 b2dd03c8-39d4-4d8f-98ff-823fe69b080e