Use case to reproduce:
 1. Open some bigger project, e.g. qtcreator.pro
 2. As soon as "Parsing C/C++ files" is reported, add a character to
    qmake's additional arguments in project mode (indexing should not be
    finished at this point).
 3. The indexing gets canceled.
 ==> ...but reindexing is not triggered.

Fix by checking whether the future was canceled.

Task-number: QTCREATORBUG-16134
Change-Id: I520c6a64a6adc1cb04cafb5e0aa56c8bf41d7b14
Reviewed-by: Christian Stenger <christian.stenger@qt.io>