Pad files with zeros when in the non-rate-limited code path
Pad files with zeros when in the non-rate-limited code path

This should make it more like the rate-limited path.  The reason I
haven't factored these two together was some recollection that the
subprocess stdin can only take file-like-objects with fileno, which
presumably it uses directly, hence, any Python-informed zero-padding
must take place on the stdout side of the mbuffer.

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

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