defined by the PLTE chunk. Such pixels decode to opaque black,
which matches what libpng does.

Fixes #4319.

On my reading, the PNG spec isn't clear whether palette index values
outside of those defined by the PLTE chunk is an error, and if not,
what to do.

Libpng 1.5.3 falls back to opaque black. png_set_PLTE says:

/* Changed in libpng-1.2.1 to allocate PNG_MAX_PALETTE_LENGTH instead
 * of num_palette entries, in case of an invalid PNG file that has
 * too-large sample values.
 */
png_ptr->palette = (png_colorp)png_calloc(png_ptr,
        PNG_MAX_PALETTE_LENGTH * png_sizeof(png_color));

ImageMagick 6.5.7 returns an error:

$ convert -version
Version: ImageMagick 6.5.7-8 2012-08-17 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2009 ImageMagick Studio LLC
Features: OpenMP
$ convert packetloss.png x.bmp
convert: Invalid colormap index `packetloss.png' @ image.c/SyncImage/3849.

R=r
CC=golang-dev
https://golang.org/cl/6822065