db_migrate: get version inside transaction.

we should never be doing two startups at once, but why take chances?  Plus,
we can then assert that all db calls are in transactions.

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

wallet/db.c

@@ -366,14 +366,16 @@ static int db_migration_count(void)
 static bool db_migrate(struct db *db)
 {
 	/* Attempt to read the version from the database */
-	int current = db_get_version(db);
-	int available = db_migration_count();
+	int current, available;
 
 	if (!db_begin_transaction(db)) {
 		/* No need to rollback, we didn't even start... */
 		return false;
 	}
 
+	current = db_get_version(db);
+	available = db_migration_count();
+
 	while (++current <= available) {
 		if (!db_exec(__func__, db, "%s", dbmigrations[current]))
 			goto fail;