When receiving a message, need to account for the fact that the displacement

of the first entry might not be the start of the user's buffer.  This is
similar to what ompi_convertor_unpack does.  This is the solution for
the test case attached to ticket #690.

Refs trac:690

This commit was SVN r13397.

The following Trac tickets were found above:
  Ticket 690 --> https://svn.open-mpi.org/trac/ompi/ticket/690

ompi/mca/mtl/base/mtl_base_datatype.h

@@ -23,6 +23,8 @@
 #include "ompi/mca/mtl/base/base.h"
 #include "ompi/constants.h"
 #include "ompi/datatype/convertor.h"
+#include "ompi/datatype/datatype.h"
+#include "ompi/datatype/datatype_internal.h"
 
 static inline int
 ompi_mtl_datatype_pack(struct ompi_convertor_t *convertor,
@@ -75,7 +77,8 @@ ompi_mtl_datatype_recv_buf(struct ompi_convertor_t *convertor,
         *free_on_error = true;
     } else {
         ompi_ddt_type_lb(convertor->pDesc, &lb);
-        *buffer = convertor->pBaseBuf + lb;
+        *buffer = convertor->pBaseBuf + lb + 
+            convertor->use_desc->desc[convertor->use_desc->used].end_loop.first_elem_disp;;
     }
     return OMPI_SUCCESS;
 }