Ensure buf->offset is always smaller than buf->size.

Ensure buf->offset is always smaller than buf->size.

Before we allowed buf->offset to be equal to buf->size. This caused an
issue where buffer_call() would call the callback twice, once for 0 bytes
at the end of the buffer, and once for len bytes at the start of the buffer.
This would cause the callback function to think the channel had encountered
an error.