[REG 3.2.2 -> 3.3.0] Crash while indexing documentation

1. Start Creator with clean settings, e.g. by passing an empty directory with the parameter "-settingspath".
2. Go to "Tools" -> "Options..." -> "Help" -> "Documentation".
3. Add qt.qch from an old Qt 4 SDK.
   Let me know if I should send it to you, it's too big to attach.
4. Click "OK".
5. Go to Help mode.
6. In one of the views of the side bar, select "Search" from the combo box.
   Creator starts indexing the documentation. While it does, it crashes.

Found by Squish test tst_HELP04.