Sledge graphical mode on OS X segfaults

With binutils 2.26 and sdl2 2.0.4 I ran:

```
$ ./sledge < tests/boot2.l
[alloc] cell heap at 0x104fe5000, 2400000 bytes reserved
[compiler] interim knows 55 symbols. enter (symbols) to see them.
[fs] mounted: /framebuffer
...
compiled def spawn-editor
compiled def spawn-repl
Segmentation fault: 11
```
