Zero is a legitimate size to be returned by bytesFree/bytesAvailable
functions, so change those functions to return some 'invalid' size
in case of an invalid drive.
This is also consistent with the original version from the Qt Systems
framework.

[ChangeLog][QtCore][QStorageInfo] Fixed sizes returned for invalid drives.

Task-number: QTBUG-45724
Change-Id: I312fba521fdf8d52d7a0ac0e46cacca625775e80
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>