Fix imbalanced calloc/MPI_free_mem.

[vehre]

coverage on master

Summary of changes

Fix imbalanced calloc/MPI_free_mem.

Rationale for changes

Memory allocated by calloc was errorneously freeed using MPI_free_mem,
which might confuse Intel MPI. This should fix a bit more of #762.

Additional info and certifications

This pull request (PR) is a:

• Bug fix
• Feature addition
• Other, Please describe:

I certify that

• I certify that:
  • I have reviewed and followed the contributing guidelines
  • I will wait at least 24 hours before self-approving the PR to give another
    OpenCoarrays developer a chance to review my proposed code
  • I have not introduced errant white space (no trailing white space or white space errors may
    be introduced)
  • I have added an explanation of what these changes do and why they should be included
  • I have checked to ensure there aren't other open Pull Requests for the same change
  • I have you written new tests for these changes
  • I have successfully tested these changes locally
  • I have commented any non-trivial, non-obvious code changes
  • The commits are logically atomic, self consistent and coherent
  • The commit messages follow best practices
  • Test coverage is maintained or increased after this is merged

Code coverage data

[everythingfunctional]

I don't fully understand why, but it works now!