chan_sip: Check that an iostream exists before accessing.

Before getting the file descriptor for an iostream check
that it is present.

ASTERISK-27534

Change-Id: Ie0aa1394007a37c30e337ea1176a6fb3a63bc99c

channels/chan_sip.c

@@ -29256,7 +29256,7 @@ static int sip_prepare_socket(struct sip_pvt *p)
 		return s->fd;
 	}
 	if ((s->type & (AST_TRANSPORT_TCP | AST_TRANSPORT_TLS)) &&
-			s->tcptls_session) {
+			s->tcptls_session && s->tcptls_session->stream) {
 		return ast_iostream_get_fd(s->tcptls_session->stream);
 	}
 	if ((s->type & (AST_TRANSPORT_WS | AST_TRANSPORT_WSS))) {