Fixes bug in enddef

Summary

While investigating an issue with hiopy on Lumi with a new YAC version, Claude found a serious bug and a couple of minor in instance.c. The issue caused interpolation objects to be reused in case grid and interpolation stack were identical even if target component was different (different processes). This result in wrong communication patterns being used.

Unfortunately, I was not able to generate a proper test of this issue.

Related Issues

• Unclear whether it is related to #249 and https://gitlab.dkrz.de/icon/icon-mpim/-/issues/445
• Closes #265 (closed)

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 version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)