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)
diff --git a/src/version.c b/src/version.c
index 9dc93f2..a0ce86d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    991,
+/**/
     990,
 /**/
     989,