fix memory leak

[mateoconlechuga]

this function leaked memory when the standard liq_image_create_rgba/liq_quantize_image/liq_write_remapped_image combo is used on large images (or forced to use conserved memory).

fixed by checking if the temporary array is already allocated by either liq_image_create_rgba and liq_write_remapped_image.

[mateoconlechuga]

I realize that not many people are still using the 2.x branch but I figured may as well push this :)

[kornelski]

Thanks for reporting this. I've fixed it in 113567c1874af0515e2f27284ae4710d0a097b26