modp base64 解码器

* MODP_B64 - High performance base64 encoder/decoder
* Version 1.3 -- 17-Mar-2006
* http://modp.com/release/base64
*
* Copyright (c) 2005, 2006  Nick Galbreath -- nickg [at] modp [dot] com
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
*   Redistributions of source code must retain the above copyright
*   notice, this list of conditions and the following disclaimer.
*
*   Redistributions in binary form must reproduce the above copyright
*   notice, this list of conditions and the following disclaimer in the
*   documentation and/or other materials provided with the distribution.
*
*   Neither the name of the modp.com nor the names of its
*   contributors may be used to endorse or promote products derived from
*   this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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