Fix a bug in the 64-bit Huffman encoder that Google discovered when encoding some...

Fix a bug in the 64-bit Huffman encoder that Google discovered when encoding some very specific (and proprietary) aerial images using quality=98, an optimized Huffman table, and the ISLOW DCT.  These images were causing the Huffman bit buffer to overflow, because the code for encoding the DC coefficient was using the equivalent of the 32-bit version of EMIT_BITS().  Thus, when 64-bit code was used, the DC coefficient code was not properly checking how many bits were in the buffer before attempting to add more bits to it.  This issue appears to have existed in all versions of libjpeg-turbo.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.4.x@1547 632fc199-4ca6-4c93-a231-07263d6284db