patch 9.0.0021: invalid memory access when adding word to spell word list

Problem:    Invalid memory access when adding word with a control character to
            the internal spell word list.
Solution:   Disallow adding a word with control characters or a trailing
            slash.
diff --git a/src/version.c b/src/version.c
index 26cb768..23798ee 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    21,
+/**/
     20,
 /**/
     19,