Fix: libpng decoding fails to detect out-of-range bKGD

Hi,

I had an OptiPNG bug report about an assertion failure occurring
because a bad bKGD sneaked into the PNG internals.

I think this is an accidental omission. The reader code fails to
detect out-of-range bKGD grayscale or truecolor values, but the writer
code is fine.

Here is the bug report (with test image) and the fix. I'm proposing to
patch all the currently-supported libpng versions, if possible.

https://sourceforge.net/p/optipng/bugs/71/

Sincerely,
Cosmin

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