[secure-transport] add ~SecureSession(void) destructor (#11553)

This commit adds a destructor for `SecureSession` which ensures the
freeing of any mbedTls allocated items. This addresses memory leaks
detected by fuzzer tests when `otInstance` is destroyed, though this
situation is unlikely in typical OpenThread stack integrations.

src/core/meshcop/secure_transport.hpp

@@ -221,6 +221,7 @@ public:
 
 protected:
     explicit SecureSession(SecureTransport &aTransport);
+    ~SecureSession(void) { FreeMbedtls(); }
 
     bool IsSessionInUse(void) const { return (mNext != this); }