Fix `LinkedList::CursorMut::pop_front` to correctly update index When `pop_front` was called while the cursor pointed to the front element, `move_next` incremented the index but it was never decremented afterwards, causing the index to incorrectly report 1 instead of 0. Always decrement the index after popping from front using `saturating_sub` to handle edge cases safely. Fixes rust-lang/rust#147616