update safety comments

Author: molpopgen

src/table_collection.rs

@@ -730,6 +730,7 @@ impl TableCollection {
         let rv = unsafe { ll_bindings::tsk_table_collection_copy(self.as_ptr(), &mut *inner, 0) };
 
         // SAFETY: we just initialized it.
+        // The C API doesn't free NULL pointers.
         handle_tsk_return_value!(rv, unsafe { Self::new_from_mbox(inner) })
     }
 

src/trees.rs

@@ -291,6 +291,7 @@ impl TreeSequence {
         };
 
         // SAFETY: we just initialized it.
+        // The C API doesn't free NULL pointers.
         handle_tsk_return_value!(rv, unsafe { TableCollection::new_from_mbox(inner) })
     }