Segfault on OS X when providing a custom formatting function

Compiling and running the following code

``` c++
#include "format.h"

struct Foo {};

void format(fmt::BasicFormatter<char> &f, const char *&, Foo) {}

int main() {
  fmt::print("{}", Foo());
}
```

results in segfault:

```
$ clang++ -std=c++11 -O1 test.cc format.cc
$ ./a.out 
Segmentation fault: 11
```

Only occurs if optimization enabled and with `-std=c++11`.
