RFR: 7419: Insecure XML parsing: XXE injection

[Evgeny Astigeevich]

This patch fixes security vulnerabilities reported in [JMC-7419](https://bugs.openjdk.java.net/browse/JMC-7419).
Testing: `man verify`

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Commit messages:
 - 7419: Insecure XML parsing: XXE injection

Changes: https://git.openjdk.java.net/jmc/pull/320/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=320&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7419
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/320.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/320/head:pull/320

PR: https://git.openjdk.java.net/jmc/pull/320