Fix change-of-sign warning found by ICC

-1 is not a valid unsigned value.

registerhandler.cpp(354): warning #68: integer conversion resulted in a change of sign
          return createIndex(row, col, TopLevelId);
                                       ^

Change-Id: I41935255704f19724d6cec16ea470e2f8f8a16a1
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>