While other comments suggest the solution is to implement the hash function based on field values, it throws away the simple, efficient, and general implementations of original memory based hash function. But if we understand the true source of the problem, isn't the obvious solution is to redefine the structure into two 64-bit fields or add in explicit padding bytes so one can explicitly zero them when necessary?

The reason for undefined behaviors is to avoid over engineering. In a capable engineer's eye, it is beautiful.