Skip em_exchange_mark without mark

Author: aycabta

lib/reline/line_editor.rb

@@ -2342,6 +2342,7 @@ def finish
   alias_method :set_mark, :em_set_mark
 
   private def em_exchange_mark(key)
+    return unless @mark_pointer
     new_pointer = [@byte_pointer, @line_index]
     @previous_line_index = @line_index
     @byte_pointer, @line_index = @mark_pointer

test/reline/test_key_actor_emacs.rb

@@ -1926,6 +1926,26 @@ def test_em_set_mark_and_em_exchange_mark
     assert_equal([0, 0], @line_editor.instance_variable_get(:@mark_pointer))
   end
 
+  def test_em_exchange_mark_without_mark
+    input_keys('aaa bbb ccc ddd')
+    assert_byte_pointer_size('aaa bbb ccc ddd')
+    assert_cursor(15)
+    assert_cursor_max(15)
+    assert_line('aaa bbb ccc ddd')
+    input_keys("\C-a\M-f", false)
+    assert_byte_pointer_size('aaa')
+    assert_cursor(3)
+    assert_cursor_max(15)
+    assert_line('aaa bbb ccc ddd')
+    assert_equal(nil, @line_editor.instance_variable_get(:@mark_pointer))
+    input_key_by_symbol(:em_exchange_mark)
+    assert_byte_pointer_size('aaa')
+    assert_cursor(3)
+    assert_cursor_max(15)
+    assert_line('aaa bbb ccc ddd')
+    assert_equal(nil, @line_editor.instance_variable_get(:@mark_pointer))
+  end
+
   def test_modify_lines_with_wrong_rs
     verbose, $VERBOSE = $VERBOSE, nil
     original_global_slash = $/