Fix crash when initializing Nokia place categories.

If network errors occur while initializing categories stale categories
were left in the category tree which can result in unbounded recursion.

Change-Id: I93a3b7a01f152473d25b492bba579b9300639bba
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>