MINOR: buffer: implement b_alloc_fast()

MINOR: buffer: implement b_alloc_fast()

This function allocates a buffer and replaces *buf with this buffer. If
no memory is available, &buf_wanted is used instead. No control is made
to check if *buf already pointed to another buffer. The allocated buffer
is returned, or NULL in case no memory is available. The difference with
b_alloc() is that this function only picks from the pool and never calls
malloc(), so it can fail even if some memory is available. It is the
caller's job to refill the buffer pool if needed.