`hc` is never NULL, since it's `hc = &chan->half[direction];`;
we really meant "is it initialized", and valgrind under CI finally
caught it:
```
==69243== Conditional jump or move depends on uninitialised value(s)
==69243== at 0x11C595: handle_local_channel_update (gossip_generation.c:758)
==69243== by 0x115254: recv_req (gossipd.c:986)
==69243== by 0x128F8D: handle_read (daemon_conn.c:31)
==69243== by 0x16BEE1: next_plan (io.c:59)
==69243== by 0x16CAE9: do_plan (io.c:407)
==69243== by 0x16CB2B: io_ready (io.c:417)
==69243== by 0x16EE1E: io_loop (poll.c:453)
==69243== by 0x1154DA: main (gossipd.c:1089)
==69243==
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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