Fix an extremely rare crash that can occur when compressing a very high-frequency...

Fix an extremely rare crash that can occur when compressing a very high-frequency MCU using quality 100 and no subsampling, and when dynamically allocating the JPEG buffer in the destination manager.  Even with a test program designed specifically to reproduce the crash, it only occurred once in about 25 million iterations.  More details here:  https://sourceforge.net/p/libjpeg-turbo/bugs/64

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1364 632fc199-4ca6-4c93-a231-07263d6284db