ctf: assert that request_sz in medium ops request_bytes is greater than 0

There's a check in medop_request_bytes to return OK if the passed
request_sz is 0.  I don't think that can ever happen, since that
request_sz comes from the max_request_sz parameter when the iterator is
created.  It wouldn't make sense to pass 0.

Change-Id: Ia2c8fb6e56195bcab607c39e366c6b58b3411f8b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>