Fix GIF save - last byte
The last byte was not saved for sure, depending on the codesize. Also if the codesize was big (12 bits which is the max) in some case an additional 0 byte was store (which is harmless but useless)
This bug only appears if the bit per code are < 8 which is possible since commit 834050c2
To trigger the bug, try to save the attached 1 pixel gif : 1PIX.GIF