Use unsafe more idiomatically

Generally, including everything that makes an unsafe block safe in the
block is good style. Since the assert! is what makes this safe, it
should go inside the block. I also added a few bits of whitespace.

src/libcore/slice.rs

@@ -303,8 +303,10 @@ impl<T> SliceExt for [T] {
     fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
         let len = self.len();
         let ptr = self.as_mut_ptr();
-        assert!(mid <= len);
+
         unsafe {
+            assert!(mid <= len);
+
             (from_raw_parts_mut(ptr, mid),
              from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
         }