Fix mismatched free for cJSON-generated string in dynsec__config_save

[jesferman]

Summary

This PR changes the deallocation of the JSON string returned by cJSON_Print() from mosquitto_free() to cJSON_free(), fixing a memory‐tracking underflow caused by mixing allocators. This underflow makes the tracking subsystem thinks that it has a lot of GB reserved, and any subsequent call to mosquitto_malloc() returns "out of memory" which leaves the broker unusable after a few operations with the Dynamic Security Plugin. The only way of recovering it is restarting Mosquitto.

Background & Problem

In dynsec__config_save(), the code currently does:

json_str = cJSON_Print(tree);
/* … */
mosquitto_free(json_str);

However:

• cJSON_Print() internally allocates with cJSON_malloc().
• Freeing that memory with mosquitto_free() (instead of cJSON_free()) decrements Mosquitto’s own allocation counter (memory_mosq.c) for a pointer that was never allocated and counted by mosquitto_malloc(), causing the counter to underflow.
• On the next mosquitto_malloc() this counter appears “too large,” causing subsequent allocations to spuriously fail with “out of memory,” rendering the broker unusable until restart.

How to reproduce

Set a memory_limit in mosquitto.conf

Using the dynamic security plugin perform a few operations of adding and/or removing users to/from ACL rules. The counter of the memory tracker will decrease with each operation until it underflows.

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