cjpeg: Fix OOB read caused by malformed 8-bit BMP

... in which one or more of the color indices is out of range for the
number of palette entries.

Fix partly borrowed from jpeg-9c.  This commit also adopts Guido's
JERR_PPM_OUTOFRANGE enum value in lieu of our project-specific
JERR_PPM_TOOLARGE enum value.

Fixes #258