Verify `Anchor::buffer_id` before resolving it or comparing it

This commit also verifies some properties about anchor resolution in the
multibuffer randomized test.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
e70b728758Antonio Scandurra committed on 1/11/2022, 3:40:12 PMparent2d5e722
2 files changedLine totals unavailable