[codec-memcache] Avoid NPE on channelInactive.

Motivation:
When no currentMessage has been set and the channel is inactive, a NPE is raised.

Modification:
Make sure that a currentMessage is available before checking the extras.

Result:
No more NPE raised potentially.

codec-memcache/src/main/java/io/netty/handler/codec/memcache/binary/AbstractBinaryMemcacheDecoder.java

@@ -201,7 +201,7 @@ public abstract class AbstractBinaryMemcacheDecoder<M extends BinaryMemcacheMess
     public void channelInactive(ChannelHandlerContext ctx) throws Exception {
         super.channelInactive(ctx);
 
-        if (currentMessage.getExtras() != null) {
+        if (currentMessage != null && currentMessage.getExtras() != null) {
             currentMessage.getExtras().release();
         }