RFR: 8293012: ConstantPool::print_on can crash if _cache is NULL

[David Holmes]

Please review this simple fix to avoid a crash when debugging the constant pool.

Testing: tier1 sanity

Thanks,
David

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Commit messages:
 - 8293012: ConstantPool::print_on can crash if _cache is NULL

Changes: https://git.openjdk.org/jdk/pull/10059/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10059&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293012
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10059.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10059/head:pull/10059

PR: https://git.openjdk.org/jdk/pull/10059