Here's the simplest example I could come up with (it's functionally equivalent to the code OP pasted):

    int zero1(int* i) {
      *i = 0;
      return 0;
    }
    
    int zero2(int* i) {
      *i = 0;
      if(i == 0) return 1;
      return 0;
    }

GCC produces the same code for these two functions. If the 'if' in zero2 is moved to the top of the function, it takes effect.

I tried this on GCC 4.8.1. A wonderful site for checking these things is: http://gcc.godbolt.org/