RFR: Only verify last object start for marked objects

[William Kemper]

In some cases, verification could invoke `size` on an unmarked object resulting in a crash.

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Commit messages:
 - Only verify last object start for marked objects

Changes: https://git.openjdk.java.net/shenandoah/pull/103/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=103&range=00
  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/103.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/103/head:pull/103

PR: https://git.openjdk.java.net/shenandoah/pull/103