gif: Prevent access to negative array indexes

It seems that a pathological gif file can cause a negative array index
to be read.  UBSAN reported this:
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]'

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