Set output vector of reduceAdd to 0 before starting to add stuff to it.

No idea why valgrind didn't complain about this.

[[Imported from SVN: r9758]]

dune/gfe/parallel/vectorcommunicator.hh

@@ -52,6 +52,7 @@ public:
     transferVector(localVector);
 
     VectorType globalVector(guIndex.nGlobalEntity());
+    globalVector = 0;
 
     for (size_t k = 0; k < globalVectorEntries.size(); ++k)
       globalVector[globalVectorEntries[k].globalIndex_] += globalVectorEntries[k].value_;