RFR: Only verify last object start for marked objects [v2]

[William Kemper]

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

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Explicitly track last marked object when verifying last object start

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/103/files
  - new: https://git.openjdk.java.net/shenandoah/pull/103/files/81e1eb0b..566d24c1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=103&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=103&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 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