parse.y: assignable_error

	* parse.y (assignable_gen): should return valid NODE always even
	  on errors.  [ruby-core:84565] [Bug #14261]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62509 b2dd03c8-39d4-4d8f-98ff-823fe69b080e