patch 9.0.0437: no error when custom completion function returns wrong type

Problem:    No error when a custom completion function returns something else
            than the expected list.
Solution:   Give an error. (closes #11100)

src/version.c

@@ -703,6 +703,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    437,
 /**/
     436,
 /**/