rastertoz.c: Fix SEGV (signal 11).

[karfas]

No description provided.

[egormanga]

Doesn't cupsRasterReadPixels() return unsigned?

[karfas]

It's years ago that I had a look on this.

As far as I remember:

• Certain values of the arguments to compress_buffer() might cause the returned min(iSize, pTarget - pBuf) to become negative (I think the semantics of the difference between two pointers are not standardized in C).
• The original code assumed a positive (unsigned) result from compress_buffer(), calculating wrong pointers and causing the SEGV later on.