PROTON-560: fix incorrect frame size calculation.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1587214 13f79535-47bb-0310-9956-ffa450edef68
(cherry picked from commit a9932a2bf2b58d89300d0a4ae63e02a7397c72c3)