That's a change in C11. It was not undefined behaviour in C99 (which was the live standard at the time).
I linked N1256, which is essentially C99 plus the errata (Tchnical Corrigendum 1, 2, & 3).
The standard did not state that at the time. Consider Defect Report #451 (as discussed in 2013 and 2014), and observe that the committee did not simply respond that the example therein is undefined behaviour.