Auto merge of #134627 - estebank:issue-133252, r=jackh726

Avoid ICE in borrowck

Provide a fallback in `best_blame_constraint` when `find_constraint_paths_between_regions` doesn't have a result. This code is due a rework to avoid the letf-over `unwrap()`, but avoids the ICE caused by the repro.

Fix #133252.

Author: bors