DSA-2575 tiff - heap-based buffer overflow

It was discovered that ppm2tiff of the TIFF tools, a set of utilities
for TIFF manipulation and conversion, is not properly checking the return
value of an internal function used in order to detect integer overflows.
As a consequence, ppm2tiff suffers of a heap-based buffer overflow.
This allows attacker to potentially execute arbitrary code via a crafted
PPM image, especially in scenarios in which images are automatically
processed.