address mpi_debug problem
Description
The mpi_debug builders randomly failed with a floating exception. The reason turned out to be an allreduce of an array which was not fully defined. Only values up to used iter were set, but the array was defined up to max_iter. The uninitialized values could randomly contain signalling nans.
News snippet
Fix mpi_debug errors (uninitialized array).
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.