Ensure we only try to wrap if handler was not removed yet. (#11455)

Motivation:
7c57c4be17 did add a way to async sign keys but did not guard against the handler been removed before try to wrap in cause of an error which could lead to a harmless NPE.

Modifications:

Add check

Result:

No more harmless NPE

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

@@ -1258,9 +1258,12 @@ public class SslHandler extends ByteToMessageDecoder implements ChannelOutboundH
                 ctx.fireUserEventTriggered(new SslHandshakeCompletionEvent(cause));
             }
 
-            // We need to flush one time as there may be an alert that we should send to the remote peer because
-            // of the SSLException reported here.
-            wrapAndFlush(ctx);
+            // Let's check if the handler was removed in the meantime and so pendingUnencryptedWrites is null.
+            if (pendingUnencryptedWrites != null) {
+                // We need to flush one time as there may be an alert that we should send to the remote peer because
+                // of the SSLException reported here.
+                wrapAndFlush(ctx);
+            }
         } catch (SSLException ex) {
             logger.debug("SSLException during trying to call SSLEngine.wrap(...)" +
                     " because of an previous SSLException, ignoring...", ex);