[Png-mng-security] patch for zero-length unknown chunk bug

[Glenn Randers-Pehrson]

After applying the patch to libpng-1.2.26, pngtest runs successfully.  It does not copy the problematic cmOD chunk because it is not copy-safe.  I have not tested with a zero-length copy-safe chunk, and I have not tested the progressive reader.

Firefox seems to be immune to this problem, although I have not tested it while using the "system" libpng instead of the embedded one, which has the buggy unknown_chunk code #ifdef'ed out.

Glenn