RFR: 7174305: (ch) Channels.newReader doesn't throw IllegalBlockingModeException if channel is configured non-blocking

[Brian Burkhalter]

Modify `sun.nio.cs.StreamDecoder::readBytes` to throw `IllegalBlockingModeException` if its channel is configured to be non-blocking.

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Commit messages:
 - 7174305: (ch) Channels.newReader doesn't throw IllegalBlockingMode if channel is configured non-blocking

Changes: https://git.openjdk.org/jdk/pull/12478/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12478&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7174305
  Stats: 91 lines in 2 files changed: 83 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12478.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12478/head:pull/12478

PR: https://git.openjdk.org/jdk/pull/12478