Fix infinite loop in commands

FIX: Fix an infinite loop caused by `insertNewlineContinueMarkup`.

Closes codemirror/dev#1502

src/commands.ts

@@ -187,7 +187,9 @@ function nonTightList(node: SyntaxNode, doc: Text) {
 function blankLine(context: Context[], state: EditorState, line: Line) {
   let insert = ""
   for (let i = 0, e = context.length - 2; i <= e; i++) {
-    insert += context[i].blank(i < e ? countColumn(line.text, 4, context[i + 1].from) - insert.length : null, i < e)
+    insert += context[i].blank(i < e
+      ? countColumn(line.text, 4, Math.min(line.text.length, context[i + 1].from)) - insert.length
+      : null, i < e)
   }
   return normalizeIndent(insert, state)
 }