Added message format error logic for EMPTY message.

Added message format error logic for EMPTY message.

According to CoAP spec(RFC 7252), Empty message has
the token length field set to Zero.
and bytes of data MUST NOT be present after the MessageID field.
If there are any bytes, they MUST be preocessed as a message
format error.

Change-Id: I1709143c9aa1851c428bbcce08d28c2d0534b89c
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3797
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>