patch 8.1.0225: mode() does not indicate using CTRL-O from Insert mode

Problem:    Mode() does not indicate using CTRL-O from Insert mode.
Solution:   Add "niI", "niR" and "niV" to mode() result. (closes #3000)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 12eac8d..40fbf37 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -8366,10 +8366,9 @@
     static void
 f_mode(typval_T *argvars, typval_T *rettv)
 {
-    char_u	buf[3];
+    char_u	buf[4];
 
-    buf[1] = NUL;
-    buf[2] = NUL;
+    vim_memset(buf, 0, sizeof(buf));
 
     if (time_for_testing == 93784)
     {
@@ -8435,6 +8434,12 @@
 	buf[0] = 'n';
 	if (finish_op)
 	    buf[1] = 'o';
+	else if (restart_edit == 'I' || restart_edit == 'R'
+							|| restart_edit == 'V')
+	{
+	    buf[1] = 'i';
+	    buf[2] = restart_edit;
+	}
     }
 
     /* Clear out the minor mode when the argument is not a non-zero number or