Fix semingly random failing assertions in DefUsesCalculator
Fix semingly random failing assertions in DefUsesCalculator

Add a default constructor to DefUse to ensure that the pointer members are
properly initialized to zero.

Change-Id: I80a1858be2662c246f0f1ecd2fa6f795bbdabf87
Reviewed-by: Erik Verbruggen <5edfe905d082749f2ac3f4e6b42937f39f083771@digia.com>

qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d