Caused a crash in CI, reproduced under valgrind by calling
any_channel_by_scid from io_poll_lightningd:
```
==2422524== Conditional jump or move depends on uninitialised value(s)
==2422524== at 0x12C98D: any_channel_by_scid (channel.c:606)
==2422524== by 0x14FF75: io_poll_lightningd (lightningd.c:682)
==2422524== by 0x225FDE: io_loop (poll.c:420)
==2422524== by 0x14A914: io_loop_with_timers (io_loop_with_timers.c:22)
==2422524== by 0x150C4E: main (lightningd.c:1193)
==2422524== Uninitialised value was created by a heap allocation
==2422524== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==2422524== by 0x234F61: allocate (tal.c:250)
==2422524== by 0x235522: tal_alloc_ (tal.c:428)
==2422524== by 0x12B500: new_unsaved_channel (channel.c:203)
==2422524== by 0x13B77A: json_openchannel_init (dual_open_control.c:2610)
==2422524== by 0x14C78D: command_exec (jsonrpc.c:630)
==2422524== by 0x14CD9F: rpc_command_hook_final (jsonrpc.c:765)
==2422524== by 0x181DDA: plugin_hook_call_ (plugin_hook.c:278)
==2422524== by 0x14D198: plugin_hook_call_rpc_command (jsonrpc.c:853)
==2422524== by 0x14D6A0: parse_request (jsonrpc.c:957)
==2422524== by 0x14DAFE: read_json (jsonrpc.c:1054)
==2422524== by 0x2231C8: next_plan (io.c:59)
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
7a7e43c078