Make countColumn robust to passing in an invalid end pos

FIX: `countColumn` no longer loops infinitely when given a `to` that's higher
than the input string's length.

Issue codemirror/dev#1502

src/column.ts

@@ -4,7 +4,7 @@ import {findClusterBreak} from "./char"
 /// taking extending characters and tab size into account.
 export function countColumn(string: string, tabSize: number, to = string.length): number {
   let n = 0
-  for (let i = 0; i < to;) {
+  for (let i = 0; i < to && i < string.length;) {
     if (string.charCodeAt(i) == 9) {
       n += tabSize - (n % tabSize)
       i++