Assert lvars_code doesn't include \n

Removing /\A.+\n/ could have an unexpected impact, depending on how
RubyLex.generate_local_variables_assign_code is implemented. It feels
like a too much assumption and the intention isn't immediately clear,
so I added these changes.

lib/irb/color.rb

@@ -156,10 +156,10 @@ def colorize_code(code, complete: true, ignore_error: false, colorable: colorabl
         end
 
         if lvars_code
-          colored.sub(/\A.+\n/, '')
-        else
-          colored
+          raise "#{lvars_code.dump} should have no \\n" if lvars_code.include?("\n")
+          colored.sub!(/\A.+\n/, '') # delete_prefix lvars_code with colors
         end
+        colored
       end
 
       private