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

[Glenn Randers-Pehrson]

I have posted two test PNGs
at http://www.simplesystems.org/users/glennrp/png_bugs

They are
zlunk-cmOD.png (the original file that Tavis supplied)

zlunk-ubSc-ubUC-ueSc-ueUC.png (a 16x16 green square with 4 unknown
zero-length chunks, safe-to-copy and unsafe-to-copy, before and
after IDAT)

Glenn