Third attempt to fix uninitialized variable in MPI::MinMaxAvg

It turned out (see cdash) that my recent commit destroyed the POD property of MinMaxAvg. Instead of changing the code that uses this property, this commit reverts 97bd7e and f402371 and instead makes sure to use initialized variables by setting the result variable first. Sorry for the initially bad solution.
Why not just write it as
```
  MinMaxAvg result = { 0,0,0,0,0 };
```
at the top of the function? You wouldn't need to initialize it with the arbitrary `in` and you'd still get an initialized object.