RFR: 8272613: CharsetDecoder.decode(ByteBuffer) throws IllegalArgumentException

[Naoto Sato]

Addressing an integer overflow on decoding a large byte buffer. Now that the code only throws OOME if necessary, describing the fact with the `@throws` tag. A CSR for that is also drafted.

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Commit messages:
 - Added OOME in the javadoc
 - cleanup
 - Added encoder tests, rename test case file
 - 8272613: CharsetDecoder.decode(ByteBuffer) throws IllegalArgumentException

Changes: https://git.openjdk.org/jdk/pull/13131/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13131&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8272613
  Stats: 97 lines in 2 files changed: 94 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13131.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13131/head:pull/13131

PR: https://git.openjdk.org/jdk/pull/13131