Ensure PNG seeks to end of previous chunk at start of load_end #5493
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Resolves #5491
If I copy existing test image bw_gradient.png, add padding to the end of the IDAT chunk and set the
MAXBLOCKso that it is the length of the original chunk (in the issue, this is triggered with the normalMAXBLOCKvalue, but changing it allows the test to use a smaller image), then Pillow reads a block of image data, and stops reading, because the image data is done. However, the IDAT chunk length indicates that should have progressed further.This leads to the next chunk being incorrectly read, which is raised as an error if
LOAD_TRUNCATED_IMAGESis enabled.This PR fixes it by reading to the end of the chunk at the start of
load_end.