b2b_logic: prevent initializing B2B session on non-INVITEs

This prevents crashes when the b2b_init_request() function is used on
methods other than INVITE or from other routes than request routes where
the INVITE SIP message is visible.

Related to #3102

(cherry picked from commit 8c0a9e0)

modules/b2b_logic/logic.c

@@ -3358,6 +3358,13 @@ str* create_top_hiding_entities(struct sip_msg* msg, b2bl_cback_f cbf,
 	struct sip_uri ct_uri;
 	int maxfwd;
 
+	if (!str_match((_str("INVITE")), &msg->first_line.u.request.method)) {
+		LM_ERR("Scenario must be initialized on INVITE but got method: %.*s\n",
+			msg->first_line.u.request.method.len,
+			msg->first_line.u.request.method.s);
+		return NULL;
+	}
+
 	if(b2b_msg_get_from(msg, &from_uri, &from_dname)< 0 ||  b2b_msg_get_to(msg, &to_uri, params->flags)< 0)
 	{
 		LM_ERR("Failed to get to or from from the message\n");
@@ -3700,6 +3707,13 @@ str* b2b_process_scenario_init(struct sip_msg* msg, b2bl_cback_f cbf,
 		goto error;
 	}
 
+	if (!str_match(&method, &msg->first_line.u.request.method)) {
+		LM_ERR("Scenario must be initialized on INVITE but got method: %.*s\n",
+			msg->first_line.u.request.method.len,
+			msg->first_line.u.request.method.s);
+		goto error;
+	}
+
 	if(b2b_msg_get_from(msg, &from_uri, &from_dname)< 0 ||
 	b2b_msg_get_to(msg, &to_uri, init_params->flags)< 0)
 	{