mgcp_protocol: assert endp when it becomes mandatory

The logic when an endp pointer is guranteed and when we are able to
process the request without the endp pointer populated is qute complex.
This shows up as a bug to coverity.

Change-Id: I1d4221f2df13c43321d5466534485cf21f0d9010
Fixes: CID#237088
diff --git a/src/libosmo-mgcp/mgcp_protocol.c b/src/libosmo-mgcp/mgcp_protocol.c
index aef41e0..e69c00f 100644
--- a/src/libosmo-mgcp/mgcp_protocol.c
+++ b/src/libosmo-mgcp/mgcp_protocol.c
@@ -1467,6 +1467,10 @@
 		return create_ok_response(NULL, 200, "DLCX", pdata->trans);
 	}
 
+	/* The logic does not permit to go past this point without having the
+	 * the endp pointer populated. */
+	OSMO_ASSERT(endp);
+
 	/* When no connection id is supplied, we will interpret this as a
 	 * wildcarded DLCX that refers to the selected endpoint. This means
 	 * that we drop all connections on that specific endpoint at once.