Reversing a string in C using pointers - purpose of if statement?

In the code below that reverses a string, I don't understand what is the purpose of the if statement. What case is this if statement trying to catch for?

Isn't this superfluous since you've already set char*end= str, and so the statement if(str) has to be true or your code would have already failed by this point?

void reverse(char* str)
{
    char *end = str;
    char temp;


 if(str){
    while (*end)
    {
        end++;
    }
}

    end--; //pulls back one for \0 character

    while (str < end)
    {
        temp = *str;
        *str++ = *end;
        *end-- = temp;
    }

}