Fix unbounded expansion of cumulative buffer in SslHandler

Related: #3567

Motivation:

SslHandler.channelReadComplete() forgets to call
super.channelReadComplete(), which discards read bytes from the
cumulative buffer.  As a result, the cumulative buffer can expand its
capacity unboundedly.

Modifications:

Call super.channelReadComplete() instead of calling
ctx.fireChannelReadComplete()

Result:

Fixes #3567

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

@@ -957,7 +957,7 @@ public class SslHandler extends ByteToMessageDecoder implements ChannelOutboundH
             ctx.read();
         }
 
-        ctx.fireChannelReadComplete();
+        super.channelReadComplete(ctx);
     }
 
     /**