BUG/MINOR: spoe: check buffer size before acquiring or releasing it

BUG/MINOR: spoe: check buffer size before acquiring or releasing it

In spoe_acquire_buffer and spoe_release_buffer, instead of checking the buffer
against buf_empty, we now check its size. It is important because when an
allocation fails, it will be set to buf_wanted. In both cases, the size is 0.

It is a proactive bug fix, no real problem was observed till now. It cannot be
backported as is in 1.7 because of all changes made on the SPOE in 1.8.