RFR: 8287178: IntegerModuloP::multiplicativeInverse returns 0 for 0

[Weijun Wang]

Add comment to the method.

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Commit messages:
 - add comment

Changes: https://git.openjdk.org/jdk/pull/9115/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287178
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9115.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9115/head:pull/9115

PR: https://git.openjdk.org/jdk/pull/9115