Make hb_object_t members private

In preparation for fixing:
https://code.google.com/p/chromium/issues/detail?id=403594
Remove this==NULL check from face->reference_table()

Not supposed to happen, and apparently this is undefined in C++.
https://code.google.com/p/chromium/issues/detail?id=403594
Rewrite this==NULL checks to avoid undefined behavior

Fixes https://code.google.com/p/chromium/issues/detail?id=403594