row_ins_check_foreign_constraint(): On timeout,
return DB_LOCK_WAIT_TIMEOUT instead of DB_LOCK_WAIT,
so that the lock wait will be properly terminated.
Also, replace some redundant assignments.

It looks like this bug was introduced in MySQL 5.7.8 by:

    commit a97f6b91227c7e0fc3151cfe5421891e79c12d19
    Author: Annamalai Gurusami <annamalai.gurusami@oracle.com>
    Date:   Tue Jun 9 16:02:31 2015 +0530

        Bug #20953265 INNODB: FAILING ASSERTION: RESULT != FTS_INVALID