updated for version 7.3.831

Problem:    Clumsy to handle the situation that a variable does not exist.
Solution:   Add default value to getbufvar() et al. (Shougo Matsushita,
            Hirohito Higashi)

src/version.c

@@ -728,6 +728,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    831,
 /**/
     830,
 /**/