io: fix input channel EOF detection

[jaysoffian]

When attempting to use parallel mode along with sudo, fabric was causing paramiko-ng to go into an infinite loop. The reason was due to a combination of fabric calling sendall() along with it misdetecting EOF on the input and closing the output channel. The fix for paramiko-ng going into an infinite loop is here:

https://github.com/ploxiln/paramiko-ng/pull/140

This commit fixes the root cause in fabric where it (apparently) erroneously detects the input channel as being closed when it is not.

[ploxiln]

hmm ... test_stdin_bytesio freezes indefinitely