gio: Prevent hang writing to a large GMemoryBuffer

gio: Prevent hang writing to a large GMemoryBuffer

Fix a hang due to overflow by using unsigned numbers and explicitly
checking if the number overflows to zero.  This also fixes the previous
logic which assigned an int which may be negative to an unsigned number
resulting in sign extension and strange results.

Use gsize rather than int to allow for large buffers on 64 bit machines.

https://bugzilla.gnome.org/show_bug.cgi?id=727988