[jdk11u-dev] RFR: 8295685: Update Libpng to 1.6.38

[Goetz Lindenmaier]

I backport this for parity with 11.0.19-oracle.

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Commit messages:
 - Backport b685fc2de4d2847de2d1efaab2890646257ea6d3

Changes: https://git.openjdk.org/jdk11u-dev/pull/1690/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1690&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295685
  Stats: 223 lines in 16 files changed: 91 ins; 35 del; 97 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1690.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1690/head:pull/1690

PR: https://git.openjdk.org/jdk11u-dev/pull/1690