Fix error when running undoSelection on empty history

Author: marijnh

lib/codemirror.js

@@ -3270,8 +3270,11 @@
 
     // Verify that there is a useable event (so that ctrl-z won't
     // needlessly clear selection events)
-    for (var i = 0; i < source.length; i++)
-      if (allowSelectionOnly || !source[i].ranges) break;
+    for (var i = 0; i < source.length; i++) {
+      event = source[i];
+      if (allowSelectionOnly ? event.ranges && !event.equals(doc.sel) : !event.ranges)
+        break;
+    }
     if (i == source.length) return;
     hist.lastOrigin = hist.lastSelOrigin = null;