I'm eyeballing src/third_party/pdfium/core/fxcodec/lgif/fx_gif.cpp. Line 138 is:

} else if (code >= code_end) {

The >= should be ==. It was == as of 2016 December, but that was changed to >= by the 1-liner CL https://codereview.chromium.org/2542673004 aka commit 9be9c34866

As per https://www.w3.org/Graphics/GIF/spec-gif89a.txt Appendix F "Variable-Length-Code LZW Compression", under the "COMPRESSION" section, the clear code is 2**<code size>, the end_of_information code is <Clear code>+1, and "The first available compression code value is <Clear code>+2". Indeed, fx_gif.cpp's CGifLZWDecoder::InitTable says:

code_clear = 1 << code_size;
code_end = code_clear + 1;

Back to the spec, codes greater than (not equal to) code_end are valid codes whose semantics are not the same as code_end, whereas the fx_gif.cpp code treats them as equivalent.

I've only read the code, not run it. I don't know how to test this, manually or automatically. IIUC, PDF per se doesn't embed GIFs. PDFium's GIF code is only included "#if defined(PDF_ENABLE_XFA)", and I'm not familiar at all with XFA.