Update invalid indices to allow for no-op loops (#577)

* Make second invalid index more negative

* Changelog

Co-authored-by: Jonas Lippuner <jlippuner@lanl.gov>

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Current develop
 
 ### Added (new features/APIs/variables/...)
+- [[PR 577]](https://github.com/lanl/parthenon/pull/577) Update invalid indices to allow for no-op loops
 - [[PR 564]](https://github.com/lanl/parthenon/pull/564) Add EstimateTimestep to particles example task list
 - [[PR 557]](https://github.com/lanl/parthenon/pull/557) Re-enable `InitMeshBlockUserData` so data can be set per-remeshing
 - [[PR 509]](https://github.com/lanl/parthenon/pull/509) Add `elapsed_main`, `elapsed_cycle`, and `elapsed_LBandAMR` functions to `Driver` as static functions to enable access to timing information in output and restart files.

src/interface/variable_pack.hpp

@@ -106,7 +106,7 @@ class PackIndexMap {
   // It will silently return invalid indices if the key doesn't exist (e.g. misspelled or
   // sparse id not part of label)
   const vpack_types::IndexPair &operator[](const std::string &key) const {
-    static const vpack_types::IndexPair invalid_indices(-1, -1);
+    static const vpack_types::IndexPair invalid_indices(-1, -2);
     auto itr = map_.find(key);
     if (itr == map_.end()) {
       return invalid_indices;