RFR: 8286397: Address possibly lossy conversions in jdk.hotspot.agent

[Ryan Ernst]

Applied required casts in jdk.hotspot.agent for the upcoming warning.
Verified by cherry-picking @asotona's patch.

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Commit messages:
 - 8286397: Address possibly lossy conversions in jdk.hotspot.agent

Changes: https://git.openjdk.org/jdk/pull/9280/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9280&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286397
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/9280.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9280/head:pull/9280

PR: https://git.openjdk.org/jdk/pull/9280