RFR: 8285493: ECC calculation error

[Weijun Wang]

Only numbers from the same modular fields can be involved in arithmetic calculations. Add `assert` to guarantee this.

Also, found one broken case and rewrote it.

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Commit messages:
 - ECC calculation error fix

Changes: https://git.openjdk.java.net/jdk/pull/8409/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8409&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285493
  Stats: 16 lines in 2 files changed: 3 ins; 4 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8409.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8409/head:pull/8409

PR: https://git.openjdk.java.net/jdk/pull/8409