Clean any remaining data previously written to temporary buffer for operations on secret keys

In S2OPC sources of the lib-specific and crypto-related functions, temporary buffer for operations on secret keys shall be cleared by setting them to zero before freed the memory.

This is the case for : (Mbed TLS)

• SOPC_KeyManager_AsymmetricKey_CreateFromBuffer (variable null_terminated_buffer)
• SOPC_KeyManager_AsymmetricKey_ToDER (variable buffer)
• SOPC_KeyManager_SerializedAsymmetricKey_CreateFromKey (variable buffer)