gif: Fix compiler warning reg. type conversion

With -Wconversion gcc complains with the following warning:

io-gif.c: In function ‘gif_read’:
io-gif.c:246:20: warning: conversion to ‘guint {aka unsigned int}’ from
‘size_t {aka long unsigned int}’ may alter its value [-Wconversion]
    context->ptr += len;
                    ^
io-gif.c:258:28: warning: conversion to ‘guint {aka unsigned int}’ from
‘size_t {aka long unsigned int}’ may alter its value [-Wconversion]
   context->amount_needed = len - (context->size - context->ptr);

This change simply changes their types to match the rhs.

https://bugzilla.gnome.org/show_bug.cgi?id=786261