fail on invalid RTP address from MGW

When the CRCX OK returns an invalid RTP address, abort the call; fixes
MSC_Tests.TC_invalid_mgcp_crash.

The original crash happened when adding this error handling without this commit
I08c03946605aa12e0a5ce8b3c773704ef5327a7a ("fsm: use deferred deallocation" for
osmo-mgw I7df2e9202b04e7ca7366bb0a8ec53cf3bb14faf3 "fix use-after-free: require
new fsm deferred dealloc, check for term"). With this error handling added,
even though avoiding a crash, the test does not pass yet, because instead of
rejecting the call, it currently composes an Assignment Command without a
Transport Layer Address. Fix that.

Change-Id: I00c3b5ff74c05bcc2b7c39375c33419916a57193

src/libmsc/msc_a.c

@@ -627,6 +627,12 @@ static void msc_a_fsm_communicating(struct osmo_fsm_inst *fi, uint32_t event, vo
 			LOG_MSC_A(msc_a, LOGL_ERROR, "Invalid data for %s\n", osmo_fsm_event_name(fi->fsm, event));
 			return;
 		}
+		if (!osmo_sockaddr_str_is_nonzero(&rtps->local)) {
+			LOG_MSC_A(msc_a, LOGL_ERROR, "Invalid RTP address received from MGW: " OSMO_SOCKADDR_STR_FMT "\n",
+				 OSMO_SOCKADDR_STR_FMT_ARGS(&rtps->local));
+			call_leg_release(msc_a->cc.call_leg);
+			return;
+		}
 		LOG_MSC_A(msc_a, LOGL_DEBUG,
 			  "MGW endpoint's RTP address available for the CI %s: " OSMO_SOCKADDR_STR_FMT " (osmux=%s:%d)\n",
 			  rtp_direction_name(rtps->dir), OSMO_SOCKADDR_STR_FMT_ARGS(&rtps->local),