RFR: 8294696 - draining buffer instead of falling back to super.transferTo

[Markus KARG]

This PR implements JDK-8294696.

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Commit messages:
 - draining buffer instead of falling back to super.transferTo

Changes: https://git.openjdk.org/jdk/pull/10525/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10525&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294696
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10525.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10525/head:pull/10525

PR: https://git.openjdk.org/jdk/pull/10525