C++ editor: Fix potential crash in completion

There was still a problem in the previous change (that's
why it was reverted) which should be fixed now.

Task-number: QTCREATORBUG-4940
Change-Id: I8a0d993b9f313fc98494cf93fed1f87939a358b1
Reviewed-on: http://codereview.qt.nokia.com/27


Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>