osc-portals4: Initialize datatype in MPI_Get_accumulate and MPI_Rget_accumulate

Fix code paths that didn't convert the MPI datatype to the
corresponding Portals4 datatype.

Thanks to Nicolas Chevalier (@shawone) for finding this bug and
submitting a patch.

ompi/mca/osc/portals4/osc_portals4_comm.c

@@ -473,6 +473,11 @@ ompi_osc_portals4_rget_accumulate(const void *origin_addr,
                 OMPI_OSC_PORTALS4_REQUEST_RETURN(request);
                 return ret;
             }
+            ret = ompi_osc_portals4_get_dt(origin_dt, &ptl_dt);
+            if (OMPI_SUCCESS != ret) {
+                OMPI_OSC_PORTALS4_REQUEST_RETURN(request);
+                return ret;
+            }
             length *= origin_count;
 
             result_md_offset = (ptl_size_t) result_addr;
@@ -835,6 +840,10 @@ ompi_osc_portals4_get_accumulate(const void *origin_addr,
             if (OMPI_SUCCESS != ret) {
                 return ret;
             }
+            ret = ompi_osc_portals4_get_dt(origin_dt, &ptl_dt);
+            if (OMPI_SUCCESS != ret) {
+                return ret;
+            }
             length *= origin_count;
 
             result_md_offset = (ptl_size_t) result_addr;