Memory leak in sixel_encoder_encode_bytes.

[muetzenmann]

By using valgrind, I found a memory leak in sixel_encoder_encode_bytes.

frame is allocated via sixel_frame_new, but never free'd.

To fix it, you can change line 1750 to

sixel_frame_t *frame = NULL;

and add

sixel_frame_unref(frame);

before the return statement.

[muetzenmann]

Just found out, that the above change fixes the memory leak, when running with valgrind.
But, without valgrind, I get a segmentation fault.

Therefore, I don't think, you should apply my change. At least test it thoroughly.