What is your definition of valid? It's not valid by any C standard or common compiler implementation. For the sake of any argument, we should at least agree on a common definition for valid.

As for deleting null checks, time-travelling UB etc. the compiler always assumes the code is valid, bug-free for the compiler-specific definition of valid. This is at least the standard definition but it can be expanded by flags and extensions. Then they optimize that code without regard to what happens if the code actually had programming bugs.