Prevent line 1 indicator sometimes displaying too high in linum-mode

[trevorpogue]

Fixes an issue first mentioned here in #17 .

• With linum-mode enabled, the line 1 indicator in the left margin would sometimes show too high up, above the actual line 1. The issue would occur when scrolling and topspace-height was zero before scrolling but greater than zero after scrolling. This prevents the issue.

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Checklist

• I have read the topspace contributing guidelines
• I have added tests (if possible) to cover my change(s)
• My changes follow the Emacs Lisp conventions and the Emacs Lisp Style Guide
• I've used the latest version of package-lint to check for packaging issues, and addressed its feedback
• The new code is not generating bytecode warnings
• I've updated the readme (if adding/changing user-visible functionality)
• I have confirmed some of these without doing them (this box shouldn't be marked)