RFR: 8294983: SSLEngine throws ClassCastException during handshake

[Kevin Driver]

Fixes a scenario where a `ServerHandshakeContext` might be cast as a `ClientHandshakeContext`.

-------------

Commit messages:
 - set consumer to null if we're not in client mode

Changes: https://git.openjdk.org/jdk/pull/13727/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13727&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294983
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13727.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13727/head:pull/13727

PR: https://git.openjdk.org/jdk/pull/13727