[pwdisswordfishy]

But if the assertion fails, the program is aborted before the pointer would have been dereferenced, making it not UB. This explanation is bogus.

[saagarjha]

Only if the assert is active. It basically means that the code is invalid when NDEBUG is set.

[quuxplusone]

When NDEBUG is set, there is no test, no assertion, at all. So yes, this code has UB if you set NDEBUG and then pass it a null pointer — but that's obvious. The code does exactly what it looks like it does; there's no tricks or time travel hiding here.