RFR: JDK-8298093: improve cleanup and error handling of awt_parseColorModel in awt_parseImage.c

[Matthias Baesken]

The cleanup and error handling in function awt_parseColorModel in awt_parseImage.c could be improved.

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Commit messages:
 - JDK-8298093

Changes: https://git.openjdk.org/jdk/pull/11508/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11508&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298093
  Stats: 11 lines in 1 file changed: 8 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11508.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11508/head:pull/11508

PR: https://git.openjdk.org/jdk/pull/11508