assert in vcl 'builder.hxx' line 404 somehow trigger a dialog on windows slaves -> hang

[Norbert Thiebaud]

All the slave bot were hung this morning....
it turned out that the assert aforementionned was triggering
and somehow pop-in up a dialog... there was dozen of these stack per box
the 'mechanism' to cancel hung build was not able to clean these up so
they stock-piled... messing up further attempt to build.

Norbert