Motivation:
SslHandlerTest tried to get access to the SslHandler in the pipeline via pipeline.get(...) which may return null if the channel was already closed and so the pipeline was teared down.
This showed up in a test run as:
```
-------------------------------------------------------------------------------
Test set: io.netty.handler.ssl.SslHandlerTest
-------------------------------------------------------------------------------
Tests run: 17, Failures: 0, Errors: 1, Skipped: 1, Time elapsed: 0.802 sec <<< FAILURE! - in io.netty.handler.ssl.SslHandlerTest
testCloseOnHandshakeFailure(io.netty.handler.ssl.SslHandlerTest) Time elapsed: 0.188 sec <<< ERROR!
java.lang.NullPointerException
at io.netty.handler.ssl.SslHandlerTest.testCloseOnHandshakeFailure(SslHandlerTest.java:640)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.lang.Thread.run(Thread.java:844)
```
Modifications:
Use an AtomicReference to propagate the SslHandler instance to the outer scope.
Result:
No more NPE.
a4393831f0