Yes, a fix inside a realloc wrapper is a defensive fix, if nothing is actually calling with the zero size in the current code base, and/or we are not yet compiling with C dialect selection that corresponds to the vandalized standard. It's an example of defensive programming, coding intended to anticipate and thwart a future problem.

Code that calls realloc(p, 0) and does not assume any one of the behaviors that are described is not incorrect according to C99.

That's par for the course in C. In C, there are situations left and right in which you're stepping on implementation defined behavior and mustn't assume a particular one.

A lot of portable coding is defensive. It can end up more portable than required. That is to say, if we consider all the platforms but the program actually ends up used over its lifetime, we can deduce that a less portable approach in some part of the code would have worked just fine.