fix crash in preview mode with --line-length=1

[jakkdl]

fixes #4062

crash happened when right_hand_split first split

def foo() -> tuple[int, int,]:  ...

to

def foo() -> tuple[
  int,
  int,
]: ...

and then left_hand_split followed up with splitting on the invisible LPAR around the return type in the first line to make it

def foo() ->
tuple[

This was due to insufficient logic in left_hand_split to abort in some weird cases, that it previously never got into because the code wasn't modified by right_hand_split beforehand - but that became a thing in #3916.

Description

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[JelleZijlstra]

Thanks! Could you add a changelog entry?

[jakkdl]

not sure how specific you wanted it, but done!