[parallel,bugfix] don't create empty MessageInformation

when we try to send data, it might happen, that for some reason we don't want to any data.
This happens, when the dynamic size of our CommPolicy is 0. In that case we must not add a
MessageInformation, otherwise we get an MPI error.

dune/common/parallel/communicator.hh

@@ -1160,7 +1160,8 @@ namespace Dune
         interfacePair != end; ++interfacePair) {
       int noSend = MessageSizeCalculator<Data,Flag>() (interfacePair->second.first);
       int noRecv = MessageSizeCalculator<Data,Flag>() (interfacePair->second.second);
-      messageInformation_.insert(std::make_pair(interfacePair->first,
+      if (noSend + noRecv > 0)
+        messageInformation_.insert(std::make_pair(interfacePair->first,
                                                 std::make_pair(MessageInformation(bufferSize_[0],
                                                                                   noSend*sizeof(typename CommPolicy<Data>::IndexedType)),
                                                                MessageInformation(bufferSize_[1],
@@ -1195,8 +1196,8 @@ namespace Dune
         interfacePair != end; ++interfacePair) {
       int noSend = MessageSizeCalculator<Data,Flag>() (source, interfacePair->second.first);
       int noRecv = MessageSizeCalculator<Data,Flag>() (dest, interfacePair->second.second);
-
-      messageInformation_.insert(std::make_pair(interfacePair->first,
+      if (noSend + noRecv > 0)
+        messageInformation_.insert(std::make_pair(interfacePair->first,
                                                 std::make_pair(MessageInformation(bufferSize_[0],
                                                                                   noSend*sizeof(typename CommPolicy<Data>::IndexedType)),
                                                                MessageInformation(bufferSize_[1],