[jdk18u] RFR: 8283469: Don't use memset to initialize members in FileMapInfo and fix memory leak

[Zhengyu Gu]

I would like to backport this patch to 18u, to fix a memory leak, also eliminates dangerous `memset` call, could potentially result in fatal crash.

The original patch does not apply cleanly, due to JDK-8261455, where it changed `FileMapInfo`'s constructor signature to pass in `full_path` as a parameter. The conflict is resolved manually by initializing `_full_path` to `nullptr`

Test:

- [x] hotspot_cds

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

Commit messages:
 - Backport d6fa8b004bcd0a2fc1015055d0177428889b4c31

Changes: https://git.openjdk.java.net/jdk18u/pull/97/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18u&pr=97&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283469
  Stats: 13 lines in 1 file changed: 8 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk18u/pull/97.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18u pull/97/head:pull/97

PR: https://git.openjdk.java.net/jdk18u/pull/97