Arena reserved size should always add ARENA_HEADER_SIZE

[FlexW]

When allocating memory with the arena the reserve size should always incorporate the size of the arena header. Otherwise, it can fail badly in arena_push().

In arena_push() on line 101 when checking if a existing free block exists the reserved size of the block is checked to see if the allocation will fit into the free block: if(new_block->res >= AlignPow2(new_block->pos, align) + size).

Little bit further down the pos_pst gets adjusted for the new free block:

    current = new_block;
    pos_pre = AlignPow2(current->pos, align);
    pos_pst = pos_pre + size;

post_pre will always be at least ARENA_HEADER_SIZE big.

This means the allocation will eventually not fit. The condition assert(pos_pst < current->res); always needs to be true. But currently that is not necessary the case.

Note: When allocating new blocks in arena_push() the ARENA_HEADER_SIZE always gets added to the reserved size as it should be.