Fix the number of items in the collation union

This doesn't change anything except make the code more clear since the
union never overflowed because C allocates the largest item in the
union. It does make the code clearer and make much more sense though.
Issue detected by coverity.

src/strings/unicode_ops.c

@@ -60,15 +60,15 @@ struct level_eval_s2 {
     MVMint32 Less, Same, More;
 };
 union level_eval_u2 {
-    MVMint32 a2[4];
+    MVMint32 a2[3];
     struct level_eval_s2 s2;
 };
 struct level_eval_s {
     union level_eval_u2 primary, secondary, tertiary, quaternary;
 };
 union level_eval_u {
     struct level_eval_s s;
-    union  level_eval_u2 a[3];
+    union  level_eval_u2 a[4];
 };
 typedef union level_eval_u level_eval;
 #define initial_stack_size 100