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
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.
2 files changed