erasure-code: assert that firefly-x/stress-split cannot load lrc

Because it would crash existing plugins.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
(cherry picked from commit d6f02d604840b448158a1470b6bb1f6d0550e97f)
Merge pull request #147 from dachary/wip-ec-feature-plugin-v2-giant

erasure-code: assert that firefly-x/stress-split cannot load lrc