check the return value of SSL_get_ex_data()

[x2018]

check the return value of SSL_get_ex_data() to prevent potential NULL dereference.
SSL_set_ex_data() called by ossl_associate_connection() may fail, so it is better to check the return value from SSL_get_ex_data(), especially data which will be dereferenced in the following code. And though sockindex seems meaningless in Curl_ssl_getsessionid() or Curl_ssl_addsessionid()?

[bagder]

If no error has been returned before, can this really happen?

And if this can happen, doesn't it signify a major error somewhere to call for everything to get closed down?

[x2018]

Yes, it looks like this can happen as

curl/lib/vtls/openssl.c

Line 4549 in 84853d9

static void ossl_associate_connection(struct Curl_easy *data,

is void type and inside it does not check the return value of SSL_set_ex_data, while SSL_set_ex_data may fail because of some internal errors.
To catch the error in time, I will change the signature of ossl_associate_connection and add several checks these days.

[x2018]

I updated the signature of ossl_associate_connection and added several checks. And I retain the usage of Curl_ssl->associate_connection() in vtls.c as before. I am not sure whether these changes make sense. Thank you for taking the time to review this PR.

[bagder]

Thanks!