fixes for Box<[T]>

The pointer in the slice must not be null, because enum representations
make that assumption. The `exchange_malloc` function returns a non-null
sentinel for the zero size case, and it must not be passed to the
`exchange_free` lang item.

Since the length is always equal to the true capacity, a branch on the
length is enough for most types. Slices of zero size types are
statically special cased to never attempt deallocation. This is the same
implementation as `Vec<T>`.

Closes #14395

src/test/run-pass/empty-allocation-non-null.rs

@@ -11,6 +11,12 @@
 pub fn main() {
     assert!(Some(box() ()).is_some());
 
+    let xs: Box<[()]> = box [];
+    assert!(Some(xs).is_some());
+
     struct Foo;
     assert!(Some(box Foo).is_some());
+
+    let ys: Box<[Foo]> = box [];
+    assert!(Some(ys).is_some());
 }