core.files.images.get_image_dimensions broken on some valid PNG images

[artscoop]

Hi,
second time I find a bug in the same function. Here, the function failed (exception raised and not caught) on a valid PNG file (this image: ​https://mega.co.nz/#!W0tXzBAK!iLprbqkVWFAzfcn2GUDRGHiH8Pxyi4DxCQi580shcec).
The error raised by PIL is a struct.error, which should be expected but is not caught by the chunk feeder loop.

The feeder knows this

            except zlib.error as e:
                # ignore zlib complaining on truncated stream, just feed more
                # data to parser (ticket #19457).
                if e.args[0].startswith("Error -5"):
                    pass
                else:
                    raise

but does not know this

from struct import error
...

except struct.error:
    pass

when you catch the proper exception, the file header fed to the parser becomes correct starting at offset 9214.
The good news is, it does not need tests.

[artscoop]

Valid but faulty image