patch 9.0.0061: ml_get error with nested autocommand

Problem:    ml_get error with nested autocommand.
Solution:   Also check line numbers for a nested autocommand. (closes #10761)
diff --git a/src/window.c b/src/window.c
index c91ebbc..e7b44ab 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6770,12 +6770,10 @@
 }
 
 /*
- * Correct the cursor line number in other windows.  Used after changing the
- * current buffer, and before applying autocommands.
- * When "do_curwin" is TRUE, also check current window.
+ * Implementation of check_lnums() and check_lnums_nested().
  */
-    void
-check_lnums(int do_curwin)
+    static void
+check_lnums_both(int do_curwin, int nested)
 {
     win_T	*wp;
     tabpage_T	*tp;
@@ -6783,22 +6781,45 @@
     FOR_ALL_TAB_WINDOWS(tp, wp)
 	if ((do_curwin || wp != curwin) && wp->w_buffer == curbuf)
 	{
-	    // save the original cursor position and topline
-	    wp->w_save_cursor.w_cursor_save = wp->w_cursor;
-	    wp->w_save_cursor.w_topline_save = wp->w_topline;
+	    if (!nested)
+	    {
+		// save the original cursor position and topline
+		wp->w_save_cursor.w_cursor_save = wp->w_cursor;
+		wp->w_save_cursor.w_topline_save = wp->w_topline;
+	    }
 
 	    if (wp->w_cursor.lnum > curbuf->b_ml.ml_line_count)
 		wp->w_cursor.lnum = curbuf->b_ml.ml_line_count;
 	    if (wp->w_topline > curbuf->b_ml.ml_line_count)
 		wp->w_topline = curbuf->b_ml.ml_line_count;
 
-	    // save the corrected cursor position and topline
+	    // save the (corrected) cursor position and topline
 	    wp->w_save_cursor.w_cursor_corr = wp->w_cursor;
 	    wp->w_save_cursor.w_topline_corr = wp->w_topline;
 	}
 }
 
 /*
+ * Correct the cursor line number in other windows.  Used after changing the
+ * current buffer, and before applying autocommands.
+ * When "do_curwin" is TRUE, also check current window.
+ */
+    void
+check_lnums(int do_curwin)
+{
+    check_lnums_both(do_curwin, FALSE);
+}
+
+/*
+ * Like check_lnums() but for when check_lnums() was already called.
+ */
+    void
+check_lnums_nested(int do_curwin)
+{
+    check_lnums_both(do_curwin, TRUE);
+}
+
+/*
  * Reset cursor and topline to its stored values from check_lnums().
  * check_lnums() must have been called first!
  */