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.

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    21,
 /**/
     20,
 /**/