gif: Preventing undefined behaviour by making left shift unsigned

io-gif.c:509:44: runtime error: index -2 out of bounds for type 'guchar [280]'
io-gif.c:510:44: runtime error: index -1 out of bounds for type 'guchar [280]'
io-gif-animation.c:422:68: runtime error: left shift of 249 by 24 places cannot be represented in type 'int'

https://bugzilla.gnome.org/show_bug.cgi?id=778584