Minor fixes according to comments

- Add Facebook copyright notice
- Make max size macros more consistent
- Fix some unchecked malloc's

contrib/educational_decoder/zstd_decompress.h

@@ -1,3 +1,12 @@
+/*
+ * Copyright (c) 2017-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
 size_t ZSTD_decompress(void *dst, size_t dst_len, const void *src,
                        size_t src_len);
 size_t ZSTD_decompress_with_dict(void *dst, size_t dst_len, const void *src,