Excelize: RIGHT() on supplementary-plane text slices with a negative index and panics

leftRight calc.go:14236 tests the length with countUTF16String, which counts a rune above U+FFFF as 2. The RIGHT branch on 14241 then slices runetext at utf8.RuneCountInStringtext-numChars, which counts it as 1. For text with N such runes the two measures are 2N and N, so any numChars between the...