lightningd: initialize error pointer in connect_activate_subd and handle_splice_abort

[Andezion]

Fixes #9442

Same uninitialized-pointer pattern as #8849 (fixed in 090f4b0 for handle_peer_spoke) also exists in:

• connect_activate_subd() in lightningd/peer_control.c
• handle_splice_abort() in lightningd/channel_control.c

In both, const u8 *error - is declared uninitialized and only conditionally set via sockpair(..., &error) before falling through to a path that dereferences it. sockpair() currently always sets the output pointer on failure, so this isn't exploitable today, but the declaration should still default to NULL defensively in case that invariant ever breaks

Fix: initialize both declarations to NULL.

Testing: compiled lightningd/channel_control.o and lightningd/peer_control.o cleanly - no behavior change on existing paths

Important

26.09 FREEZE August 5th: Non-bugfix PRs not ready by this date will wait for 26.12.

RC1 is scheduled on August 17th

The final release is scheduled for September 7th.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
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