Prevent overread when decoding malformed JPEG

The accelerated Huffman decoder was previously invoked if there were
> 128 bytes in the input buffer.  However, it is possible to construct a
JPEG image with Huffman blocks > 430 bytes in length
(http://stackoverflow.com/questions/2734678/jpeg-calculating-max-size).
While such images are pathological and could never be created by a
JPEG compressor, it is conceivable that an attacker could use such an
artifially-constructed image to trigger an input buffer overrun in the
libjpeg-turbo decompressor and thus gain access to some of the data on
the calling program's heap.

This patch simply increases the minimum buffer size for the accelerated
Huffman decoder to 512 bytes, which should (hopefully) accommodate any
possible input.

This addresses a major issue (LJT-01-005) identified in a security audit
by Cure53.

ChangeLog.txt

@@ -39,6 +39,15 @@ set to JMSG_COPYRIGHT.
 [10] Fixed a bug in the build system that was causing the Windows version of
 wrjpgcom to be built using the rdjpgcom code.
 
+[11] Fixed an issue in the accelerated Huffman decoder that could have caused
+the decoder to read past the end of the input buffer when a malformed,
+specially-crafted JPEG image was being decompressed.  In prior versions of
+libjpeg-turbo, the accelerated Huffman decoder was invoked (in most cases) only
+if there were > 128 bytes of data in the input buffer.  However, it is possible
+to construct a JPEG image in which a single Huffman block is over 430 bytes
+long, so this version of libjpeg-turbo activates the accelerated Huffman
+decoder only if there are > 512 bytes of data in the input buffer.
+
 
 1.2.1
 =====

jdhuff.c

@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * Modifications:
- * Copyright (C) 2009-2011, D. R. Commander.
+ * Copyright (C) 2009-2011, 2016, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains Huffman entropy decoding routines.
@@ -743,7 +743,7 @@ decode_mcu_fast (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
  * this module, since we'll just re-assign them on the next call.)
  */
 
-#define BUFSIZE (DCTSIZE2 * 2)
+#define BUFSIZE (DCTSIZE2 * 8)
 
 METHODDEF(boolean)
 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)