RFR: 8286389: Address possibly lossy conversions in jdk.crypto.ec

[Ryan Ernst]

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

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Commit messages:
 - 8286389: Address possibly lossy conversions in jdk.crypto.ec

Changes: https://git.openjdk.org/jdk/pull/9203/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9203&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286389
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9203.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9203/head:pull/9203

PR: https://git.openjdk.org/jdk/pull/9203