Auto merge of #150076 - heathdutton:fix-linked-list-cursor-pop-front-index, r=Mark-Simulacrum

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

Author: bors