Fix buffer shrinking logic in UTCP.

Fix buffer shrinking logic in UTCP.

Don't set the maximum size to low values; keep it at the minimum of the
previous size or of the default maximum size. If it is set to something
smaller than one MTU, this would prevent receiving packets from the
peer, and channel traffic would not progress and not close properly.

We also don't move memory when shrinking the internal buffer, so we
should keep the size large enough so the last byte in the buffer is
covered when the offset is non-zero.