patch 7.4.2021

Problem:    Still too many buf_valid() calls.
Solution:   Make au_new_curbuf a bufref.  Use bufref_valid() in more places.

src/version.c

@@ -758,6 +758,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2021,
 /**/
     2020,
 /**/