Keep capacity when unsplit on empty other buf

[Gbillou]

When trying to unsplit() from a previous split():

• if the other buf is not empty the unsplit() results in a buf with the initial capacity
• if the other buf is empty, other is dropped, capacity lost, and anything new added to the initial buf leads to a reallocation

This changes this second behavior so that even if other is empty the overall capacity is kept.

[Darksonn]

Thanks.