[IE8] Javascript error when selecting toolbar

[matsu]

To reproduce:

1. Select bold from toolbar.
2. Write messages.
3. Select all messages and cut.
4. Select underline from toolbar.
5. Write messages.
6. Paste messages cut before between underline messages and underline messages.
7. Select all messages.

Results: Javascript error occur when selecting underline from toolbar.

[Krzysztof Studnik]

short test case:

1. Select bold from toolbar.
2. Write text.
3. Select all and cut.
4. Select underline from toolbar.
5. Paste

Result:

<p>
	<strong><u><strong>Test</strong></u></strong></p>

• When cursor is set in bold-underlined text, i cannot disable bold either underline
• switch to source and back -> Bold and Underline buttons are disabled

1. with cursor set in the middle of text click few times on underline resilt as on attached screen.

Above happened only once, i cannot reproduce it again when browser cache is enabled, no error is displayed

[Garry Yao]

DUP of #5756.