[splinecommon_] Use idx instead of i

Otherwise the reverse feature would not work.
This bug exists since the beginning of splinecommon_.

Thanks Timo for spotting a mistake I'd introduced.
5 jobs for !2897 with feature/fix-splinecommon_.hh
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-latest-release-clang

passed full-dune-latest-release-gcc

passed full-dune-master-gcc

passed minimal-dune-latest-release-gcc

 
  Downstream Modules
passed trigger lecture