Make sure that TGeoManager exists before each call to gGeoManager
(call gGeoManager via a function which performs a test). 
This fixes a crash in VMC examples when the VMC application builds geometry via TVirtualMC and then selects TGeant3TGeo as MC.


git-svn-id: http://root.cern.ch/svn/root/trunk@34810 27541ba8-7e3a-0410-8455-c3a389f83636