fix position potentially off if rune is not ascii

lex/lex.go

@@ -175,7 +175,7 @@ func (L *Lexer) Next() (ctx TokenContext) {
 	for {
 		ctx.Position = L.pos
 		if L.useBuf {
-			ctx.Position -= 1
+			ctx.Position -= utf8.RuneLen(L.buf)
 		}
 		ctx.Token = L.next()
 		if ctx.Token == nil {