mp_obj_print_exception(): Assert that traceback has sane number of entries.

py/obj.c

@@ -52,6 +52,7 @@ void mp_obj_print_exception(mp_obj_t exc) {
         machine_uint_t n, *values;
         mp_obj_exception_get_traceback(exc, &n, &values);
         if (n > 0) {
+            assert(n % 3 == 0);
             printf("Traceback (most recent call last):\n");
             for (int i = n - 3; i >= 0; i -= 3) {
 #if MICROPY_ENABLE_SOURCE_LINE