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: https://v8.googlecode.com/svn/branches/bleeding_edge@18378 ce2b1a6d-e550-0410-aec6-3dcde31c8c00