wallet: fix erroneous allocation of db_col_optional_scid.

We are supposed to allocate of the ctx we're passed, not tmpctx.

Doesn't matter for now, because we don't use this result with anything which outlives tmpctx,
but we're going to:

```
==47574==ERROR: AddressSanitizer: heap-use-after-free on address 0x6040005a8f38 at pc 0x55d3c584d252 bp 0x7ffddfb1b090 sp 0x7ffddfb1b088
READ of size 8 at 0x6040005a8f38 thread T0
    #0 0x55d3c584d251 in json_add_closed_channel /home/runner/work/lightning/lightning/lightningd/closed_channel.c:27:3
    #1 0x55d3c584ca5a in json_listclosedchannels /home/runner/work/lightning/lightning/lightningd/closed_channel.c:118:3
    #2 0x55d3c58c0cbe in command_exec /home/runner/work/lightning/lightning/lightningd/jsonrpc.c:808:8
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

wallet/wallet.c

@@ -1672,7 +1672,7 @@ static struct short_channel_id *db_col_optional_scid(const tal_t *ctx,
 	if (db_col_is_null(stmt, colname))
 		return NULL;
 
-	scid = tal(tmpctx, struct short_channel_id);
+	scid = tal(ctx, struct short_channel_id);
 	*scid = db_col_short_channel_id(stmt, colname);
 	return scid;
 }