Problem: Crash when using ":mkspell" with an empty .dic file.
Solution: Check for an empty word tree.
6669de1b23
Co-authored-by: Bram Moolenaar <Bram@vim.org>
Problem: Not enough characters accepted for 'spellfile'.
Solution: Add vim_is_fname_char() and use it for 'spellfile'.
bc49c5f48f
Cherry-pick related doc update from Vim runtime.
Co-authored-by: Bram Moolenaar <Bram@vim.org>