RFR: 8253948: Memory leak in ImageFileReader

[Zhengyu Gu]

ImageFileReader allocates ImageModuleData in ImageFileReader::open(), but never free.

Also renamed module_data to _module_data to be consistent with other member variables.

Test:
- [x] tier1 on Linux x86_64

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Commit messages:
 - 8253948: Memory leak in ImageFileReader

Changes: https://git.openjdk.java.net/jdk/pull/490/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=490&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253948
  Stats: 15 lines in 2 files changed: 10 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/490.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/490/head:pull/490

PR: https://git.openjdk.java.net/jdk/pull/490