Bugfix

I had to change 
```
#if SHOW_ERROR_LINE
  for(i=0; i<=sc->file_i; i++) {
    if (sc->load_stack[sc->file_i].kind & port_file) {
      fname = sc->load_stack[i].rep.stdio.filename;
      if(fname)
        sc->free(fname);
    }
  }
#endif
```

in scheme.c

to 

`if(fname && strlen(fname))`

Else it crashes, if fname is not a Null pointer but it is not set i.e. frame = ""
