Assign to the specified variable the two-byte, two's complement integer (in host byte order) comprised of the two bytes of this stream at the current cursor location (in network byte order), update the cursor location, and return a reference to this stream. If this stream is initially invalid, this operation has no effect. If this function otherwise fails to extract a valid value, this stream is marked invalid and the value of variable is undefined. Note that the value will be sign-extended.