tokio: check page capacity before obtaining base pointer (#4731)

This doesn't cause any issues in practice because this is a private API
that is only used in ways that cannot trigger UB. Indexing into `slots`
is not sound until after we've asserted that the page is allocated,
since that aliases the first slot which may not be allocated. This PR
also switches to using `as_ptr` to obtain the base pointer for clarity.

Co-authored-by: David Koloski <dkoloski@google.com>

tokio/src/util/slab.rs

@@ -551,10 +551,9 @@ impl<T> Slots<T> {
     fn index_for(&self, slot: *const Value<T>) -> usize {
         use std::mem;
 
-        let base = &self.slots[0] as *const _ as usize;
-
-        assert!(base != 0, "page is unallocated");
+        assert_ne!(self.slots.capacity(), 0, "page is unallocated");
 
+        let base = self.slots.as_ptr() as usize;
         let slot = slot as usize;
         let width = mem::size_of::<Slot<T>>();