Remove useless negative check for unsigned variables

salad/src/list.c:154:12: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]

```c
if(index < 0 || index > sld_list_size(list))
```

&&

salad/src/vector.c:146:12:

```c
if(index < 0 || index >= vector->object_count)
```