Don't crash on closed channel

As seen in production:

    java.nio.channels.CancelledKeyException: null
        at sun.nio.ch.SelectionKeyImpl.ensureValid(SelectionKeyImpl.java:73)
        at sun.nio.ch.SelectionKeyImpl.readyOps(SelectionKeyImpl.java:87)
        at java.nio.channels.SelectionKey.isWritable(SelectionKey.java:312)
        at io.github.erikvanzijst.scalatlsproxy.TlsProxyHandler.process(TlsProxyHandler.scala:175)
        at io.github.erikvanzijst.scalatlsproxy.TlsProxy.run(TlsProxy.scala:28)

src/main/scala/io/github/erikvanzijst/scalatlsproxy/TlsProxyHandler.scala

@@ -152,7 +152,7 @@ class TlsProxyHandler(selector: Selector, clientChannel: SocketChannel, config:
           }.get
 
       if (phase == Response)
-        if (clientKey.isWritable) {
+        if (clientKey.isValid && clientKey.isWritable) {
           serverBuffer.flip()
           clientChannel.write(serverBuffer)
           serverBuffer.compact()
@@ -180,7 +180,7 @@ class TlsProxyHandler(selector: Selector, clientChannel: SocketChannel, config:
       }
 
       if (phase == Error)
-        if (clientKey.isWritable) {
+        if (clientKey.isValid && clientKey.isWritable) {
           serverBuffer.flip()
           clientChannel.write(serverBuffer)
           serverBuffer.compact()