[png-mng-implement] Avoid serializing PLTE chunks larger than allowed by bit_depth

[Cosmin T. <ct...@gm...>]

Hi,

The PNG spec mentions in Section 11.2.3 (PLTE):

"The number of palette entries shall not exceed the range that can be
represented in the image bit depth (for example, 2^4 = 16 for a bit
depth of 4)."

However, libpng has always allowed reading and writing invalid images
of this type. I noticed it after analyzing a bug report filed against
OptiPNG on Debian.

See the test case "crash.png" at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=787647

I can, of course, patch OptiPNG to disallow it, independently of
libpng. However, libpng should take care of it also.

The libpng patch is available in my fork at BitBucket:
https://bitbucket.org/ctruta/libpng-cos/commits/4998f0af89091f758456805563d6cf96ea661b35

Sincerely,
Cosmin