RFR: 8304836: Make MALLOC_MIN4 macro more robust

[Naoto Sato]

The current macro assumes the argument is `jint` because all locations pass `jint`. However, this would not work if a wider type such as `jlong` is passed. Removing the `unsigned` cast and make the condition explicit would make the macro more robust.

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Commit messages:
 - 8304836: Make MALLOC_MIN4 macro more robust

Changes: https://git.openjdk.org/jdk/pull/13564/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304836
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13564.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13564/head:pull/13564

PR: https://git.openjdk.org/jdk/pull/13564