What's the purpose of old_i? It looks like it's never used for anything.
Other than than, the patch looks good to me.
I'd guess that the "if (i < 0)" was simply optimized away. This isn't
necessarily a compiler bug: if I understand correctly, a strict reading of
the C standards says it's legitimate for a compiler to assume that code is
written in such a way that signed-arithmetic overflow never happens, and
gcc (for one) is known to take advantage of this.
Also, it would be nice to cleanup the whitespace in this function while
you're fixing it; at the moment it's showing me a mixture of tabs and
spaces.