Check whether `repeated_read_cmd` returned nil and did not actually run nvim or 
it did, but still returned nil for whatever reason.

This variant uses `fdopen()` which is not standard, but it fixes problem on my 
system. In next commit `scriptin` will use `FileDescriptor*` from os/fileio in 
place of `FILE*`.

Problem: as fileio is cached and reads blocks this is going to wait
until either EOF or reading enough characters to fill rbuffer. This is
not good when reading user input from stdin as script.

Assume something with system() if second test hangs as well. Assume something 
with reading stdin if not.