patch 8.2.0403: when 'buftype' is "nofile" there is no overwrite check

Problem:    When 'buftype' is "nofile" there is no overwrite check.
Solution:   Also check for existing file when 'buftype' is set.
            (closes #5807)

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    403,
 /**/
     402,
 /**/