Fix panic on link close

[plorenz]

Panic likely caused by link closing because payload channel is closed while ack channel is still being initialized.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1a62428]

goroutine 5241 [running]:
github.com/openziti/channel/v2.(*channelImpl).Close(0xc00269a540?)
        /home/plorenz/go/pkg/mod/github.com/openziti/channel/v2@v2.0.130/impl.go:214 +0xe8
github.com/openziti/ziti/router/xlink_transport.(*splitImpl).Close(0xc000efac60)
        /home/plorenz/work/nf/ziti/router/xlink_transport/xlink_split.go:108 +0x83
github.com/openziti/ziti/router/handler_link.(*closeHandler).HandleClose.func1.1()
        /home/plorenz/work/nf/ziti/router/handler_link/close.go:51 +0x24
created by github.com/openziti/ziti/router/handler_link.(*closeHandler).HandleClose.func1 in goroutine 2950
        /home/plorenz/work/nf/ziti/router/handler_link/close.go:50 +0x3b6