fix memory leak in esp_mqtt_client_set_uri(): free memory before strdup() (IDFGH-15563)

[FurTrader]

Repeated calls to esp_mqtt_client_set_uri were leaking 32 bytes of ram each time.

Solution is to free the memory used by password and username.

Description

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Testing

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Checklist

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• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
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