JAMES-3715 Prevent possible NPE upon inactive SMTP channel

[chibenwa]

java.lang.NullPointerException: null
         at org.apache.james.protocols.netty.BasicChannelInboundHandler.channelInactive(BasicChannelInboundHandler.java:137)
         at org.apache.james.smtpserver.netty.SMTPChannelInboundHandler.channelInactive(SMTPChannelInboundHandler.java:64)
         at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:262)
         at io.netty.channel.AbstractChannelHandlerContext.access$300(AbstractChannelHandlerContext.java:61)
         at io.netty.channel.AbstractChannelHandlerContext$4.run(AbstractChannelHandlerContext.java:253)
         at io.netty.util.concurrent.DefaultEventExecutor.run(DefaultEventExecutor.java:66)
         at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)
         at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
         at java.base/java.lang.Thread.run(Unknown Source)