Fix iscached for not-yet-registered data

runtime/starpu/codelets/codelet_dataflush.c

@@ -47,6 +47,9 @@ int RUNTIME_desc_iscached(const MORSE_desc_t *A, int Am, int An)
     starpu_data_handle_t *ptrtile = (starpu_data_handle_t*)(A->schedopt);
     ptrtile += ((int64_t)(A->lmt) * (int64_t)An + (int64_t)Am);
 
+    if (!*ptrtile)
+        return 0;
+
     return starpu_mpi_cached_receive(*ptrtile);
 }
 #endif