Fix bug in es-mda where returned variable could potentially be unbound

[oyvindeide]

Posterior context could potentially be unbound, so return the prior context instead. The last thing that happens in the loop is to set prior_context = posterior_context, so the result should be the same.

Issue
Resolves #4754

Approach
Short description of the approach

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.
• Updated documentation
• Ensured new behaviour is tested

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[pinkwah]

Supert!