Strange mapping at 0 on Mach-O

This is with `master` as of now on OSX. If you create a test as described [here](https://github.com/google/pprof/issues/130#issuecomment-357979679). The code will go to `symbolizer.go` `newMapping()`. It will first call `obj.Open("myexe", 0x100000000, ...)` which is correct because the `__TEXT` segment is mapped there. However it then also tries to call `obj.Open("myexe", 0, ...` which fails because the code assumes you are trying to symbolise things in the `__TEXT` segment. As far as I can see the only thing mapped to 0 is the `__PAGEZERO` segment.

I'm not really sure what is going on here, but something isn't right. It also doesn't seem to cause any obvious issues but maybe I am overlooking something.