Fix OOB Read when saving GIF of xsize=1

[wiredfool]

Triggered in Tests/test_file_gif.py::test_optimize

There is a OOB read of size 1 when the width of the image is 1.

Valgrind output:

Tests/test_file_gif.py::test_optimize **70** 
**70** **********************************************************************
**70** Tests/test_file_gif.py::test_optimize
**70** **********************************************************************
==70== Invalid read of size 1
==70==    at 0x87A7194: ImagingGifEncode (GifEncode.c:237)
==70==    by 0x877D519: _encode (encode.c:141)
==70==    by 0x64BF47: method_vectorcall_VARARGS (descrobject.c:300)