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.
Yes, that is probably better. I used max and -max to make sure we are on the extreme end in any case.