Check length of msg_new before trying to encode a copy of the IE into
it. The code path for IE_AOIP_TRASP_ADDR is different since it gets
replaced. With the libosmocore patch in Depends below the
gsm0808_enc_aoip_trasp_addr function checks the length.
Depends: libosmocore I632986b99d841abff0f14c6da65f030175f5c4a1
Fixes: Coverity CID#273004
Change-Id: I1fc4c81e139bab3d7d977ef9467f62d8088884db