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/autocmd.c b/src/autocmd.c
index 841da14..c376f20 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -2209,9 +2209,13 @@
 	    ap->last = FALSE;
 	ap->last = TRUE;
 
+	// Make sure cursor and topline are valid.  The first time the current
+	// values are saved, restored by reset_lnums().  When nested only the
+	// values are corrected when needed.
 	if (nesting == 1)
-	    // make sure cursor and topline are valid
 	    check_lnums(TRUE);
+	else
+	    check_lnums_nested(TRUE);
 
 	save_did_emsg = did_emsg;