STYLE: Coverity 1080963: Dereference after null check in TreeIteratorBase

The instance variable m_Tree should never be NULL as a precondition of the
Add() method, and doesn't need to be explicitly checked for NULLness. A
TreeIteratorBase requires that m_Tree be set upon instantiation, and quite
sensibly, no other method bothers to verify that it is non-NULL.

Change-Id: I1208de1639b7c8b3261b3b55344152b75f32c587
Merge topic '1080963'

e19df2e4 STYLE: Coverity 1080963: Dereference after null check in TreeIteratorBase