Problem: Computed previous buffer line count may be beyond end of
buffer. This results in signs being removed from `b_signcols`
that were never included in it, tripping an assertion.
Solution: Store the previous line count as it was before appending or
deleting lines. Use it to clamp the edited region when
clearing signs before a splice, after which it is reset.
(cherry picked from commit 4a706a7092)
b8e5fd51fd