This didn't take into account the size of the header plus the size of
the allocation itself by virtue of `align_forward_uint`; this could
result in no change if `size` was equal to `b.alignment` because the
number is aligned, and if `actual_size` and `size` ended up being equal,
no additional space would be requested.
This meant that a block would end up being allocated on top of its
buddy's head.
Fixes#3435
57e2d8f1dd