lightningd: fix compile error in peer_control

Error on gcc 7.3.0:

lightningd/peer_control.c: In function ‘json_close’:
lightningd/peer_control.c:955:3: error: ‘channel’ may be used uninitialized in
  this function [-Werror=maybe-uninitialized]
   channel_set_state(channel,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
       channel->state, CHANNELD_SHUTTING_DOWN);

Signed-off-by: William Casarin <jb55@jb55.com>

lightningd/peer_control.c

@@ -901,7 +901,7 @@ static struct command_result *json_close(struct command *cmd,
 {
 	const jsmntok_t *idtok;
 	struct peer *peer;
-	struct channel *channel;
+	struct channel *channel = NULL;
 	unsigned int *timeout;
 	bool *force;