Fix mosquitto_loop_start() leaving the mosq struct in an invalid state

This occurs if thread creation fails.

Closes #3496. Thanks to ehoffman2.

ChangeLog.txt

@@ -1,7 +1,7 @@
 2.1.3 - 2026-02-xx
 ==================
 
-# Broker:
+# Broker
 - Fix MOSQ_EVT_DISCONNECT being called before MOSQ_EVT_ACL_CHECK for the will
   of that client. Closes #3487.
 
@@ -9,6 +9,10 @@
 - Fix potential crash if reading a file in restricted mode and the group id
   does not have an entry in /etc/groups. Closes #3498.
 
+# Lib
+- Fix mosquitto_loop_start() leaving the mosq struct in an invalid state if
+  thread creation fails. Closes #3496.
+
 # Plugins
 - Fix migrate_to_persist_sqlite.py not base64 decoding message payloads when
   migrating. Closes #3492.
@@ -20,11 +24,11 @@
 2.1.2 - 2026-02-09
 ==================
 
-Broker:
+# Broker
 - Forbid running with `persistence true` and with a persistence plugin at the
   same time.
 
-Build:
+# Build
 - Build fixes for OpenBSD. Closes #3474.
 - Add missing libedit to docker builds. Closes #3476.
 - Fix static/shared linking of libwebsockets under cmake.

lib/thread_mosq.c

@@ -48,7 +48,6 @@ int mosquitto_loop_start(struct mosquitto *mosq)
 		return MOSQ_ERR_INVAL;
 	}
 
-	mosq->threaded = mosq_ts_self;
 	if(!COMPAT_pthread_create(&mosq->thread_id, NULL, mosquitto__thread_main, mosq)){
 #if defined(__linux__)
 		pthread_setname_np(mosq->thread_id, "mosquitto loop");
@@ -57,6 +56,7 @@ int mosquitto_loop_start(struct mosquitto *mosq)
 #elif defined(__FreeBSD__) || defined(__OpenBSD__)
 		pthread_set_name_np(mosq->thread_id, "mosquitto loop");
 #endif
+		mosq->threaded = mosq_ts_self;
 		return MOSQ_ERR_SUCCESS;
 	}else{
 		return MOSQ_ERR_ERRNO;