accept MGCP without SDP

SDP is an optional part of MGCP messages. Do not fail when there is no SDP part.

Practically this is useful to compose simpler MGCP responses from TTCN3 tests.
osmo-mgw itself always includes SDP, so there is no real impact on operating
libosmo-mgcp-client with osmo-mgw from osmo-bsc or osmo-msc.

Change-Id: I608001626459ea72415fb142f857550bbb90c683
3 files changed