use checked_add for upper bound

Author: nyurik

library/core/src/iter/adapters/intersperse.rs

@@ -200,10 +200,10 @@ where
         lo.saturating_sub(!started as usize)
             .saturating_add(next_is_some as usize)
             .saturating_add(lo),
-        hi.map(|hi| {
+        hi.and_then(|hi| {
             hi.saturating_sub(!started as usize)
                 .saturating_add(next_is_some as usize)
-                .saturating_add(hi)
+                .checked_add(hi)
         }),
     )
 }