Fix ChannelManager.closeChannel NPE, close AsyncHttpClient#681

Author: Stephane Landelle

providers/netty/src/main/java/org/asynchttpclient/providers/netty/channel/ChannelManager.java

@@ -307,14 +307,14 @@ public void close() {
     }
 
     public void closeChannel(Channel channel) {
-        removeAll(channel);
-        Channels.setDiscard(channel);
 
         // The channel may have already been removed if a timeout occurred, and
         // this method may be called just after.
         if (channel != null) {
             LOGGER.debug("Closing Channel {} ", channel);
             try {
+                removeAll(channel);
+                Channels.setDiscard(channel);
                 channel.close();
             } catch (Throwable t) {
                 LOGGER.debug("Error closing a connection", t);