mruby-array-ext: add type check in __product_group to prevent crash

the internal method __product_group assumes all elements in the arys
argument are Arrays, but when called directly (e.g., via send or fuzzing),
non-array values can cause segfault. add type check before accessing with
RARRAY_LEN to convert crash to proper TypeError.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-array-ext/src/array.c

@@ -1360,6 +1360,7 @@ ary_product_group(mrb_state *mrb, mrb_value self_ary)
   while (j > 0) {
     j -= 1;
     mrb_value a = RARRAY_PTR(arys_ary)[j]; // arys[j]
+    mrb_check_type(mrb, a, MRB_TT_ARRAY);
     mrb_int b = RARRAY_LEN(a);             // a.size
     mrb_ary_set(mrb, group, j + 1, RARRAY_PTR(a)[n % b]);
     n /= b;