Fix issue #21 - Decoding fails to often

Loop from 'i' to 'M' instead of 'i' to 'L' during Gaussian elimination in the
second phase.

Signed-off-by: Anders Martinsson <anders.martinsson@intinor.se>

Backport/manually cherry-picked from commit:
dea18e54



Signed-off-by: Luca Fulchir <luker@fenrirproject.org>