RFR: Do not coalesce dead objects in global cycle if GC was cancelled

[William Kemper]

Attempting to coalesce and fill with an incomplete mark context triggers an assert in debug builds (and may cause problems for a release build).

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Commit messages:
 - Do not coalesce dead objects in global cycle if GC was cancelled

Changes: https://git.openjdk.java.net/shenandoah/pull/89/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=89&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/89.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/89/head:pull/89

PR: https://git.openjdk.java.net/shenandoah/pull/89