image/gif: revert incorrect usage of clear

CL 570555 replaced a loop which added empty
color.RGBA elements with a call to clear.

color.Palette is a slice of interfaces, so using
clear results in a slice of nil elements, rather
than what we previously had which was empty
color.RGBA elements. This could cause a panic when
attempting to re-encode a GIF which had an
extended color palette because of the weird
transparency hack.

This was discovered by OSS-Fuzz. I've added a test
case using their reproducer in order to prevent
future regressions.

Change-Id: I00a89257d90b6cca68672173eecdaa0a24f18d9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/577555
Reviewed-by: Nigel Tao <nigeltao@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
2 files changed
tree: 0817dafdef6255ea682b7ae40335dc89de5cdc54
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. go.env
  13. LICENSE
  14. PATENTS
  15. README.md
  16. SECURITY.md
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