erb.rb: preserve the behavior for invalid syntax

	comment. Fix regression at r58948.

	I even don't want to deprecate it because deprecation needs to lex all
	embedded Ruby script using Ripper and it would be slow.  So Let me just
	keep this behavior of Ruby 2.4. No change is the best compatibility.

	This commit stopped using String#-@ because it's harmful for "ambiguous
	first argument" warning if we really want to maintain this behavior.

	[Bug #14243]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@61581 b2dd03c8-39d4-4d8f-98ff-823fe69b080e