[jdk13u-dev] RFR: 7131823: bug in GIFImageReader

[Yuri Nesterenko]

Backporting this fix for parity with LTS releases. Clean ports; jdk/javax/imageio tests pass.

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Commit messages:
 - Backport a31130fd4056907edcb420761722c629a33273eb

Changes: https://git.openjdk.org/jdk13u-dev/pull/393/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk13u-dev&pr=393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7131823
  Stats: 175 lines in 2 files changed: 158 ins; 4 del; 13 mod
  Patch: https://git.openjdk.org/jdk13u-dev/pull/393.diff
  Fetch: git fetch https://git.openjdk.org/jdk13u-dev pull/393/head:pull/393

PR: https://git.openjdk.org/jdk13u-dev/pull/393