Problem:- This crash happens because of thd = NULL , and while checking
for wsrep_on , we no longer check for thd != NULL (MDEV-7955). So this
problem is regression of MDEV-7955. However this patch not only solves
this regression , It solves all regression caused by MDEV-7955 patch.

To get all possible cases when thd can be null , assert(thd)/
assert(trx->mysql_thd) is place just before all wsrep_on and innodb test
suite is run. And the assert which caused failure are removed with a physical
check for thd != NULL. Rest assert are removed. Hopefully this method will
remove all current/potential regression of MDEV-7955.