Merge pull request #23763 from neovim/backport-23760-to-release-0.9

[Backport release-0.9] fix(mkspell): prevent Unicode character overflow

src/nvim/spellfile.c

@@ -3127,7 +3127,7 @@ static int spell_read_dic(spellinfo_T *spin, char *fname, afffile_T *affile)
     // Remove CR, LF and white space from the end.  White space halfway through
     // the word is kept to allow multi-word terms like "et al.".
     l = (int)strlen(line);
-    while (l > 0 && line[l - 1] <= ' ') {
+    while (l > 0 && (uint8_t)line[l - 1] <= ' ') {
       l--;
     }
     if (l == 0) {