Fix crash on editing qmake path

Invalid qmake's would lead to crashes

Task-Nr: QTCREATORBUG-5544

Change-Id: Iba3d4004a7d242f0ff5e56556befa82b58e68bbb
Reviewed-on: http://codereview.qt.nokia.com/2277


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>