channels: check for null

Change-Id: Id5df8a835428019ac0e11e7c722ee5d554c74a3a

src/jamidht/channeled_transfers.cpp

@@ -38,8 +38,10 @@ ChanneledOutgoingTransfer::ChanneledOutgoingTransfer(const std::shared_ptr<Chann
 ChanneledOutgoingTransfer::~ChanneledOutgoingTransfer()
 {
     channel_->setOnRecv({});
+    if (file_)
         file_->setOnRecv({});
     channel_->shutdown();
+    if (file_)
         file_->close();
 }