Fix IRB crash when typing string literal with control/meta sequence (#1182)

Author: tompng

lib/irb/ruby-lex.rb

@@ -107,6 +107,7 @@ def should_continue?(tokens, line, line_num)
             t.type != :NEWLINE &&
             t.type != :EOF
           trailing = line.byteslice(t.location.end_column..)
+          trailing ||= '' # in case end_line is wrong (e.g. `"\C-`)
           break
         end
       end

test/irb/test_irb.rb

@@ -828,6 +828,12 @@ def test_dynamic_prompt_with_blank_line
         assert_dynamic_prompt(input_with_prompt)
       end
 
+      def test_dynamic_prompt_with_incomplete_ctrl_meta_sequence_string
+        assert_dynamic_prompt([['001:0:":* ', %q("\C-)]])
+        assert_dynamic_prompt([['001:0:":* ', %q("\M-)]])
+        assert_dynamic_prompt([['001:0:":* ', %q("\C-\M-)]])
+      end
+
       def assert_dynamic_prompt(input_with_prompt)
         expected_prompt_list, lines = input_with_prompt.transpose
         def @irb.generate_prompt(opens, continue, line_offset)