o_char and e_char are always empty, then returncode is None

[cheops]

by line 282 o_char and e_char are always empty
since while o_char != b'': and while e_char != b'': will have done their job

when this break is executed, it might be that p.poll() has not yet filled in the returncode
This might result in all kind of strange behavior if the result of spawn is checked and found to be None in stead of 0

Same concern for the break for prompt.
maybe after o_buf = read() a call should be added p.wait()
to ensure the child process has terminated?