RFR: JDK-8263559: Add missing initializers to VM_PopulateDumpSharedSpace

[Thomas Stuefe]

Trivial.

Sonarcloud reports missing initializers in VM_PopulateDumpSharedSpace. I don't see how this could lead to an error, since these members are initialized later, but for cleanliness sake lets initialize them.

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/2996/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2996&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263559
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2996.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2996/head:pull/2996

PR: https://git.openjdk.java.net/jdk/pull/2996