fix for compiler warning (ATLASG-1503)
As ATLASG-1503 keeps showing up in the Egamma dashboard... I propose this most minimal modification of AnaToolHandle.icc in order to avoid the warning (+ possible unwanted compiler optimisation) and close the ticket.
Expected impact: none, as the ToolHandle operators * and -> currently have identical implementation (ToolHandle.icc) and both actually raise an exception if the tool pointer is null.
Tagging expert @krumnack.