Error paths dereference an uninitialized pointer

[daywalker90]

The exact same pattern (const u8 *error; declared uninitialized, later conditionally set via sockpair(..., &error) before goto/fall-through to an error-send path) as in #8849 also exists at

1. lightningd/peer_control.c:1380 (connect_activate_subd())
2. lightningd/channel_control.c:331

as reported by @Andezion