The megablock allocator does not currently check that after aligning the free
region if it still has enough space to actually do the allocation.
This causes it to return a memory region which it didn't fully allocate
itself. Even worse, it can cause it to return a block with a region
that will be present in two allocation pools.
This causes if you're lucky an error from the OS that you're committing
memory that has never been reserved, or causes random heap corruption.
This change makes it consider the alignment as well.