[jdk8u-dev] RFR: 8305815: Update Libpng to 1.6.39 [v3]

[Martin Balao]

> Hi all,
> 
> This pull request contains a backport of commit [c1f759e9d01d646eac69442452151b0467eab306](https://github.com/openjdk/jdk11u-dev/commit/c1f759e9d01d646eac69442452151b0467eab306) from the [openjdk/jdk11u-dev](https://github.com/openjdk/jdk11u-dev) repository.
> 
> This backport is a follow up fix of [JDK-8295685](https://bugs.openjdk.org/browse/JDK-8295685).
> 
> This backport applied cleanly, except for the changes to the THIRD_PARTY_README files that are specific to 8u.
> 
> Thanks!

Martin Balao has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  Backport c1f759e9d01d646eac69442452151b0467eab306

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

Changes: https://git.openjdk.org/jdk8u-dev/pull/373/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=373&range=02
  Stats: 129 lines in 17 files changed: 29 ins; 8 del; 92 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/373.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/373/head:pull/373

PR: https://git.openjdk.org/jdk8u-dev/pull/373