Fixes for premature exit from decoding

[pieper]

Need to change errors to warnings to read sample file with this code:

https://github.com/covictory/pngdump

https://github.com/covictory/pngdump/blob/master/testsite/cxr16bit.png

[kripken]

Thanks @pieper , should these be submitted upstream perhaps? (they don't seem specific to emscripten at first glance)

[pieper]

Hi - good question - I didn't try but could investigate.

I noticed a note that something similar had been done in other parts of the libpng code. Maybe this is just in the way error handling happens in emscripten leads to an abort instead of just a message.

I'm rebuilding now with this project's build script and can see what happens for different libpng versions.