Fix fOffset calculation with rule applying to the whole object.

If a read rule does not a specific target, the corresponding StreamerElement
has a name that is not one of the data member (past or present) and thus
they need to be handled explicitly in TBranchElement::InitializeOffset.
A new 'bit' (kWholeObject) is introduced to explicitly distinguish those
type of StreamerElement.

(cherry picked from v5-34-00-patches commit db8ea8103f1cbc6bc7d514f33a1c02261ec0512c)