RETURN SHOULD BE 3.

net_length_size return incorrectly the buffer size when the input
length is 251. This is because we use one byte for length less than
251 and three bytes for length above these value. The value 251 is
reserved for NULL and this encoding requires 3 bytes.
The patch fixes the issue by return correct value for net_length_size
when the input size is 251.

Change-Id: If11ac2171170ad03830b8520eabc2f71d8a5d5c3