res_xmpp: Don't crash when trying to send a message without a connection

If we never establish a connection to our Jabber server, iksemel never sets up
its internal transport pointer, so attempting to send a message dereferences a
NULL pointer and causes a crash.

ASTERISK-21855 #close
Reported by: Jeremy Kister

Change-Id: I204a568894e4a53ab929783ecc594a000f04d79c

res/res_xmpp.c

@@ -2564,10 +2564,16 @@ static void xmpp_log_hook(void *data, const char *xmpp, size_t size, int incomin
 static int xmpp_client_send_raw_message(struct ast_xmpp_client *client, const char *message)
 {
 	int ret;
-#ifdef HAVE_OPENSSL
-	int len = strlen(message);
 
+	if (client->state == XMPP_STATE_DISCONNECTED) {
+		/* iks_send_raw will crash without a connection */
+		return IKS_NET_NOCONN;
+	}
+
+#ifdef HAVE_OPENSSL
 	if (xmpp_is_secure(client)) {
+		int len = strlen(message);
+
 		ret = SSL_write(client->ssl_session, message, len);
 		if (ret) {
 			/* Log the message here, because iksemel's logHook is