invoice: move link to the top of the structure.

memleak doesn't detect pointers to within an object, only pointers to their
exact address (it's simpler this way).  Moving the linked list to the
top of the structure means it can follow the chain.

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

lightningd/invoice.h

@@ -17,9 +17,9 @@ enum invoice_status {
 };
 
 struct invoice {
+	struct list_node list;
 	u64 id;
 	enum invoice_status state;
-	struct list_node list;
 	const char *label;
 	u64 msatoshi;
 	struct preimage r;