Add assertion to prevent closed channel transfer to spawned process

Now, if i specify *custom_channels* for *gaffer.process.Process* i can receive something like that:

```
[1]    10182 segmentation fault (core dumped)  thriftserver

Program terminated with signal 11, Segmentation fault.
#0  0x00007f776f2b4c67 in uv__process_init_stdio (container=0x7f775c04fb70, 
    fds=0x7f775c062bd0, writable=1) at src/unix/process.c:146
```

In this request i add assertion to prevent closed channel passing to spawning process.