cugraph_c ensure host memory has finished copy before giving to user

Previously we just staged the copy onto the stream, with no promise
that the copy would be completed by the time the caller dereferences
it.

cpp/src/c_api/array.cpp

@@ -356,6 +356,7 @@ extern "C" cugraph_error_code_t cugraph_type_erased_device_array_view_copy_to_ho
                       reinterpret_cast<byte_t const*>(internal_pointer->data_),
                       internal_pointer->num_bytes(),
                       p_handle->handle_->get_stream());
+    p_handle->handle_->sync_stream();
 
     return CUGRAPH_SUCCESS;
   } catch (std::exception const& ex) {