#16 Fix LZW decompression for full code table

[Bert Münnich]

Version 4.1.6 and 4.2.1 do not decode this image correctly: http://ge.tt/15VJnNB/v/52

This is caused by the fact, that DGifDecompressLine() overwrites the last entry of the code table again and again in the case that the table is full.

The attached patch solves this issue and applies cleanly against both mentioned versions.

This issue was originally reported here:
https://github.com/muennich/sxiv/issues/75