Fix bounds issue in unicode collation (though not possible to trigger)

This can't be triggered since we don't have anywhere near 10 graphemes
in a collation rule, but change it anyway. Since we increase the value
of i immediately after the conditional, that could make i == 10 for an
array with only 10 elements and cause it to go past its bounds.

src/strings/unicode_ops.c

@@ -408,7 +408,7 @@ static MVMint64 collation_push_cp (MVMThreadContext *tc, collation_stack *stack,
                      last_good_result = -1;
             MVMint64 i, result = query;
             DEBUG_PRINT_SUB_NODE(main_nodes[query]);
-            for (i = 0; result != -1 && MVM_string_ci_has_more(tc, ci) && i < 10;) {
+            for (i = 0; result != -1 && MVM_string_ci_has_more(tc, ci) && i < 9;) {
                 i++;
                 /* Only grab a codepoint if it doesn't already exist in the array */
                 if (cp_num <= i) {