fixes issue in interp_operator_sum_mvp_at_tgt

Summary

fixes avoid use-after-free of tgt_field in sum_mvp_at_tgt async get

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successful
• Update changelog, if changes are worth mentioning (Develop section in doc/src/changelog.dox, add section if not yet included)