passing pointers of zero-size arrays to redists
The addresses of arrays passed to xt_redist_s_exchange can be NULL, when the size of the array is zero. This can cause problems especially when redist collections are used.
For zero-sized arrays a redist should not contain any message. When this happens we might have to consider applying some kind of special handling.
When the Fortran interface is used, no appropriate C_LOC-pointer can be generated.
- reproduce problem with test (only occurs with certain MPIs)
- fix problem of define that NULL pointers are not allowed