OpenSslEngine may use networkBIO after calling shutdown

Motivation:
To ensure that all bytes queued in OpenSSL/tcnative internal buffers we invoke SSL_shutdown again to stimulate OpenSSL to write any pending bytes. If this call fails we may call SSL_free and the associated shutdown method to free resources. At this time we may attempt to use the networkBIO which has already been freed and get a NPE.

Modifications:
- Don't call bioLengthByteBuffer(networkBIO) if we have called shutdown() in ReferenceCountedOpenSslEngine

Result:
Fixes netty#6466

handler/src/main/java/io/netty/handler/ssl/ReferenceCountedOpenSslEngine.java

@@ -34,7 +34,6 @@
 
 import java.nio.ByteBuffer;
 import java.nio.ReadOnlyBufferException;
-import java.security.AlgorithmConstraints;
 import java.security.Principal;
 import java.security.cert.Certificate;
 import java.util.ArrayList;
@@ -479,9 +478,11 @@ public final SSLEngineResult wrap(
                     // It is possible when the outbound was closed there was not enough room in the non-application
                     // buffers to hold the close_notify. We should keep trying to close until we consume all the data
                     // OpenSSL can give us.
-                    boolean didShutdown = doSSLShutdown();
+                    if (!doSSLShutdown()) {
+                        return newResultMayFinishHandshake(NOT_HANDSHAKING, 0, bytesProduced);
+                    }
                     bytesProduced = bioLengthBefore - SSL.bioLengthByteBuffer(networkBIO);
-                    return newResultMayFinishHandshake(didShutdown ? NEED_WRAP : NOT_HANDSHAKING, 0, bytesProduced);
+                    return newResultMayFinishHandshake(NEED_WRAP, 0, bytesProduced);
                 }
 
                 // Flush any data that may be implicitly generated by OpenSSL (handshake, close, etc..).
@@ -1044,6 +1045,10 @@ public final synchronized void closeOutbound() {
         }
     }
 
+    /**
+     * Attempt to call {@link SSL#shutdownSSL(long)}.
+     * @return {@code false} if the call to {@link SSL#shutdownSSL(long)} was not attempted or returned an error.
+     */
     private boolean doSSLShutdown() {
         if (SSL.isInInit(ssl) != 0) {
             // Only try to call SSL_shutdown if we are not in the init state anymore.