[crac] RFR: 8375286: [CRaC] Crash with empty CRaCCheckpointTo

[Radim Vansa]

When the JVM is started with `-XX:CRaCCheckpointTo=` (empty value), on checkpoint the code accesses null pointer and the process crashes.

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Commit messages:
 - 8375286: [CRaC] Crash with empty CRaCCheckpointTo

Changes: https://git.openjdk.org/crac/pull/287/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375286
  Stats: 157 lines in 5 files changed: 150 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/crac/pull/287.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/287/head:pull/287

PR: https://git.openjdk.org/crac/pull/287