mpi: Fix MPIMsg construction in presence of Bundles

Author: FabioLuporini

devito/mpi/routines.py

@@ -1217,7 +1217,7 @@ def _arg_defaults(self, allocator, alias, args=None):
 
     def _arg_values(self, args=None, **kwargs):
         # Any will do
-        for f in self.target.handles:
+        for f in self.target.components:
             try:
                 alias = kwargs[f.name]
                 break