Fix a couple of issues with return value checking. JFREAD(), which wraps fread(...

Fix a couple of issues with return value checking.  JFREAD(), which wraps fread(), will never return -1.  fread() will instead return 0 or a short object count if an error occurs, and ferror() will return 1 in that case.  The second issue was that we were assigning the return value of ftell() to an unsigned long prior to checking the value, so the value would never be < 0 if an error occurred.  It would instead be (unsigned long)-1.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1060 632fc199-4ca6-4c93-a231-07263d6284db