RFR: 8283469: Don't use memset to initialize members in FileMapInfo and fix memory leak

[Zhengyu Gu]

`FileMapInfo` uses `memset` in its constructor to initialize data member, it is generally a bad idea. It can break things badly if adds a new member with a non-trivial constructor and `memset` is not suitable.

Besides, it leaks malloc'd `_header` member.

Test:

- [x]  hotspot_cds

-------------

Commit messages:
 - 8283469: Don't use memset to initialize members in FileMapInfo and fix memory leak

Changes: https://git.openjdk.java.net/jdk/pull/7899/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7899&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283469
  Stats: 11 lines in 1 file changed: 5 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7899/head:pull/7899

PR: https://git.openjdk.java.net/jdk/pull/7899