fix return type

src/datawizard/memalloc.c

@@ -774,7 +774,7 @@ starpu_allocate_buffer_on_node(unsigned dst_node, size_t size)
 #endif
 
 	if (_starpu_memory_manager_can_allocate_size(size, dst_node) == 0)
-		return NULL;
+		return 0;
 
 #ifdef STARPU_DEVEL
 #warning TODO: we need to use starpu_malloc which should itself inquire from the memory manager is there is enough available memory