lightningd: fix overzealous memleak detection in json_add_datastore.

The lifetime is bound to the response, but memleak gets upset if it
sees it.  Use the standard take() pattern insteas.

```
lightningd-5 2024-11-28T05:02:27.503Z **BROKEN** lightningd: MEMLEAK: 0x55b58f329338
lightningd-5 2024-11-28T05:02:27.503Z **BROKEN** lightningd:   label=common/utils.c:150:char[]
lightningd-5 2024-11-28T05:02:27.504Z **BROKEN** lightningd:   alloc:
lightningd-5 2024-11-28T05:02:27.585Z **BROKEN** lightningd:     ccan/ccan/tal/tal.c:488 (tal_alloc_)
lightningd-5 2024-11-28T05:02:27.586Z **BROKEN** lightningd:     ccan/ccan/tal/tal.c:517 (tal_alloc_arr_)
lightningd-5 2024-11-28T05:02:27.586Z **BROKEN** lightningd:     ccan/ccan/tal/tal.c:861 (tal_dup_)
lightningd-5 2024-11-28T05:02:27.586Z **BROKEN** lightningd:     common/utils.c:150 (utf8_str)
lightningd-5 2024-11-28T05:02:27.589Z **BROKEN** lightningd:     lightningd/datastore.c:23 (json_add_datastore)
lightningd-5 2024-11-28T05:02:27.589Z **BROKEN** lightningd:     lightningd/datastore.c:205 (json_datastore)
lightningd-5 2024-11-28T05:02:27.590Z **BROKEN** lightningd:     lightningd/jsonrpc.c:808 (command_exec)
lightningd-5 2024-11-28T05:02:27.590Z **BROKEN** lightningd:     lightningd/jsonrpc.c:954 (rpc_command_hook_final)
lightningd-5 2024-11-28T05:02:27.590Z **BROKEN** lightningd:     lightningd/plugin_hook.c:196 (plugin_hook_call_next)
lightningd-5 2024-11-28T05:02:27.591Z **BROKEN** lightningd:     lightningd/plugin_hook.c:183 (plugin_hook_callback)
lightningd-5 2024-11-28T05:02:27.593Z **BROKEN** lightningd:     lightningd/plugin.c:663 (plugin_response_handle)
lightningd-5 2024-11-28T05:02:27.593Z **BROKEN** lightningd:     lightningd/plugin.c:775 (plugin_read_json_one)
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/datastore.c

@@ -20,9 +20,9 @@ static void json_add_datastore(struct json_stream *response,
 
 		json_add_u64(response, "generation", generation);
 		json_add_hex(response, "hex", data, tal_bytelen(data));
-		str = utf8_str(response, data, tal_bytelen(data));
+		str = utf8_str(NULL, data, tal_bytelen(data));
 		if (str)
-			json_add_string(response, "string", str);
+			json_add_string(response, "string", take(str));
 	}
 }