Valgrind error on test_xpay_fake_channeld

I was testing flakiness of xpay test `test_xpay_fake_channeld`, when I found this valgrind error:
```
Valgrind error file: valgrind-errors.356233
==356233== Conditional jump or move depends on uninitialised value(s)
==356233== at 0x1116ED: handle_block_added (xpay.c:3159)
==356233== by 0x11D7A8: ld_command_handle (libplugin.c:2144)
==356233== by 0x11DB8A: ld_read_json (libplugin.c:2282)
==356233== by 0x15DAC1: next_plan (io.c:60)
==356233== by 0x15DF4C: do_plan (io.c:422)
==356233== by 0x15E005: io_ready (io.c:439)
==356233== by 0x15F99B: io_loop (poll.c:470)
==356233== by 0x11DFD6: plugin_main (libplugin.c:2481)
==356233== by 0x11876E: main (xpay.c:3412)
==356233==
{

Memcheck:Cond
fun:handle_block_added
fun:ld_command_handle
fun:ld_read_json
fun:next_plan
fun:do_plan
fun:io_ready
fun:io_loop
fun:plugin_main
fun:main
}

```