[PATCH] Check for overflow in DroplessArena and return aligned pointer

[PATCH] Check for overflow in DroplessArena and return aligned pointer

* Check for overflow when calculating the slice start & end position.
* Align the pointer obtained from the allocator, ensuring that it
  satisfies user requested alignment (the allocator is only asked for
  layout compatible with u8 slice).
* Remove an incorrect assertion from DroplessArena::align.

Gbp-Pq: Name u-c0c3327ade8b0caa3043425ee45842c86fbd4f76.patch