b2b_logic: Fix a logical bug which could cause crashes

... in b2b_init_request().  Completes c84fe37.

Neither @e1 or @e2 can be NULL after the "if" guard, otherwise there is
a good chance of a crash shortly afterwards.

(cherry picked from commit dec3809)

modules/b2b_logic/logic.c

@@ -2896,7 +2896,7 @@ str* b2b_process_scenario_init(struct sip_msg* msg, b2bl_cback_f cbf,
 		goto error;
 	}
 
-	if (!e1 && !e2) {
+	if (!e1 || !e2) {
 		LM_ERR("Two bridge entities required!\n");
 		goto error;
 	}