Fix regression in ModUnionTable RAM usage

We forgot to check if a reference changed before assigning it back,
this caused extra dirty pages in the image.

BusinessCard .art PSS
Before: 1678 KB
After: 991 KB

System wide .art PSS goes down ~20MB on my shamu.

Bug: 27906566

(cherry picked from commit 014885a8)

Change-Id: I64d149f10f3ef1ed1cb61810282559b47823014c