erasure-code: preload fails if < 0

And not if < -1.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2495 from dachary/wip-erasure-code-preload

erasure-code: preload fails if < 0

Reviewed-by: Sage Weil <sage@redhat.com>