openssl: check if sessionid flag is enabled before retrieving session

openssl: check if sessionid flag is enabled before retrieving session

Ideally, Curl_ssl_getsessionid should not be called unless sessionid
caching is enabled. There is a debug assertion in the function to help
ensure that. Therefore, the pattern in all vtls is basically:

  if(primary.sessionid) {lock(); Curl_ssl_getsessionid(...); unlock();}

There was one instance in openssl.c where sessionid was not checked
beforehand and this change fixes that.

Prior to this change an assertion would occur in openssl debug builds
during connection stage if session caching was disabled.

Reported-by: Jim Beveridge
Fixes https://github.com/curl/curl/issues/8472
Closes https://github.com/curl/curl/pull/8484

Change-Id: I720345c0c456f7375b5b842846061c043e236a57