rdppm.c: Fix buf overrun caused by bad binary PPM This extends the fix in 1e81b0c3ea26f4ea8f56de05367469333de64a9f to include binary PPM files with maximum values < 255, thus preventing a malformed binary PPM input file with those specifications from triggering an overrun of the rescale array and potentially crashing cjpeg, TJBench, or any program that uses the tjLoadImage() function. Fixes #433