popen: remove redundant fd check before perform IO

The function already checks flags to find out whether the file
descriptor should be available for reading / writing. When it is so, the
corresponding fd is great or equal to zero.

The further commits will add missed diagnostics for IO functions and it
is hard to write a meaningful error message for a situation that is not
possible. Moreover, we would obligated to document the error as one of
possible failures in a function contract (while it can't occur).

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>