Check the size of the g_new arguments

Check the size of the g_new arguments

We're passing integers without validating their size, and newer GCC are
very cross about it, with warnings like:

    warning: argument 1 range [18446744071562067968, 18446744073709551615]
    exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]

We should check we're not overflowing the allocation size, by limiting
the range of values we can use.

First of all, we need to use `gsize` instead of a random `int`, since we're
allocating data.

Additionally, we need to check that the multiplication that computes the
size of the allocation doesn't overflow the maximum value of a `gsize`.