Guard GIF LZW decoder against next_code overflow#329
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… clear code Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
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Some GIFs (about 1 in 10 from Giphy's search API, per #319) contain LZW data that keeps assigning new codes without ever sending a clear code. The decoder kept incrementing next_code past the 12-bit maximum, overflowing the u13 and panicking in decode. This stops adding dictionary entries once the table is full (4096 entries) and treats the unrecoverable KwKwK case at a full table as invalid data, which gif.zig already maps to InvalidData. Added a regression test that feeds such a stream and previously crashed at the same line as the report.
Fixes #319