mp_obj_instance_make_new: avoid undefined behavior

If kw_args is NULL then memcpy() gets a NULL source argument.
This is undefined behavior under the C standard, even if 0 bytes
are being copied.

This problem was found using clang 7's scan-build static analyzer.

Author: jepler

py/objtype.c

@@ -336,8 +336,10 @@ mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self, size_t n_args, cons
             mp_obj_t *args2 = m_new(mp_obj_t, 1 + n_args + 2 * n_kw);
             args2[0] = MP_OBJ_FROM_PTR(self);
             memcpy(args2 + 1, args, n_args * sizeof(mp_obj_t));
-            // copy in kwargs
-            memcpy(args2 + 1 + n_args, kw_args->table, 2 * n_kw * sizeof(mp_obj_t));
+            if (n_kw) {
+                // copy in kwargs
+                memcpy(args2 + 1 + n_args, kw_args->table, 2 * n_kw * sizeof(mp_obj_t));
+            }
             new_ret = mp_call_function_n_kw(init_fn[0], n_args + 1, n_kw, args2);
             m_del(mp_obj_t, args2, 1 + n_args + 2 * n_kw);
         }