Fix assertion in CodeLocation constructor.
Fix assertion in CodeLocation constructor.

When loading a build graph, we need to take care not to use the non-
default constructor for objects that were originally default-
constructed. Otherwise we will pass in an empty file path, which is not
allowed.

Change-Id: I32830f51749fd8d5a4662f4e6ed1d227de859193
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

Distrotech/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs,qt-labs/qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,gatzka/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs