MemoryMappedFile segfaults

```
#0  mono_mmap_open_file (path=0x0, path_length=-2800, mode=0, mapName=0x110063798, mapName_length=4195344, capacity=0x110302f90, access=271593360, options=0, ioerror=0xb, error=0x3) at file-mmap-posix.c
```

Looks like there's some bogus information going on.... mapName seems right, but not much else?

Might be marshal related....