Avoid SLOW_ASSERT when calling HeapGraphNode::GetChildrenCount
It may occur that GetChildrenCount is called on the node which has no children and stored last in the internal nodes array. In that case HeapEntry::children_arr() would fail when taking address of the element at index children_index_ which is past the last element in the children's array.
BUG=None
LOG=N
R=alph@chromium.org, ulan@chromium.org
Review URL: https://codereview.chromium.org/
112623005
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18378
ce2b1a6d-e550-0410-aec6-
3dcde31c8c00