RFR: JDK-8295685: Update Libpng to 1.6.38 [v5]

[Harshitha Onkar]

> Updated libpng library to v1.6.38.
> 
> Following files have been updated -
> 
> - 12 source code files of libpng 
> - LICENSE, README, CHANGES, UPDATING text files
> - legal/libpng.md

Harshitha Onkar has updated the pull request incrementally with one additional commit since the last revision:

  text updated

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10953/files
  - new: https://git.openjdk.org/jdk/pull/10953/files/6fb39567..3dc49a58

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10953&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10953&range=03-04

  Stats: 7 lines in 1 file changed: 0 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10953.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10953/head:pull/10953

PR: https://git.openjdk.org/jdk/pull/10953