patch 8.0.0991: using wrong character conversion for DBCS

Problem:    Using wrong character conversion for DBCS.
Solution:   Use utf_char2bytes instead of mb_char2bytes. (Yasuhiro Matsumoto,
            closes #2012)

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    991,
 /**/
     990,
 /**/