RFR: 8266014: Regression brought by optimization done with JDK-4926314

[Brian Burkhalter]

Please consider this request to correct a minor problem with the optimization added for JDK-4926314. The change is to attempt to read the number of elements remaining in the target buffer unless that number is non-positive in which case read zero. The test addition fails without and passes with the implementation change.

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Commit messages:
 - 8266014: Regression brought by optimization done with JDK-4926314

Changes: https://git.openjdk.java.net/jdk/pull/3708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266014
  Stats: 20 lines in 2 files changed: 16 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3708.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3708/head:pull/3708

PR: https://git.openjdk.java.net/jdk/pull/3708