RFR: Check explicitly for affiliation when updating references

[William Kemper]

It's possible for a mutator allocation to 'activate' a region before setting the affiliation, which would trip the assert here (or worse, miss the assert and then go off the rails).

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Commit messages:
 - Check explicitly for affiliation when updating references

Changes: https://git.openjdk.java.net/shenandoah/pull/47/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=47&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/47.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/47/head:pull/47

PR: https://git.openjdk.java.net/shenandoah/pull/47