Fix safety comment

The size assertion in the comment was inverted compared to the code. After fixing that the implication that `(new_size >= old_size) => new_size != 0` still doesn't hold so explain why `old_size != 0` at this point.

library/std/src/alloc.rs

@@ -166,8 +166,9 @@ impl System {
  match old_layout.size() {
  0 => self.alloc_impl(new_layout, zeroed),
 
- // SAFETY: `new_size` is non-zero as `old_size` is greater than or equal to `new_size`
- // as required by safety conditions. Other conditions must be upheld by the caller
+ // SAFETY: `new_size` is non-zero as `new_size` is greater than or equal to `old_size`
+ // as required by safety conditions and the `old_size == 0` case was handled in the
+ // previous match arm. Other conditions must be upheld by the caller
  old_size if old_layout.align() == new_layout.align() => unsafe {
  let new_size = new_layout.size();