g_mbedtls_slot_cb is not tested before being called

[jd-imi]

I migrated from v3.5.1 to v3.7.9 and I fell calling a null function (I didn't call atca_register_ecdh_slot_cb before calling mbedtls_ecdh_gen_public). I assume that troubleshooting this issue could be easier.

Here is the call to the function: https://github.com/MicrochipTech/cryptoauthlib/blob/7f00156/lib/mbedtls/atca_mbedtls_ecdh.c#L69

In version 3.5.1, atca_mbedtls_ecdh_slot_cb was called instead of g_mbedtls_slot_cb which need to be registered.

[Srinivas-E]

Hi @jd-imi ,
With v3.7.9 this approach is modifed. Yes, we have to register a callback and use it.
https://github.com/MicrochipTech/cryptoauthlib/blob/main/test/integration/test_mbedtls.c#L284

Similarly, atca_unregister_ecdh_slot_cb() is added.