Fix sess_hits counter on the server

[samuel40791765]

Issues:

Addresses CryptoAlg-2700

Description of changes:

While debugging the Ruby tests using an external session cache for TLS 1.2, there was an issue found where we were bumping up the SSL_CTX_sess_hits counter even when there wasn't a session found in the cache. We bump sess_hits in three places, due to it's broader implied meaning in OpenSSL. @WillChilds-Klein has an older commit with a nice description of the counter: 4b18065

Apparently session could be a nullptr, which means a session wasn't found. In the original code, this meant that we would be updating the cache hit counter regardless of a session being found or not. This should only be updated when a session was found. Changing it to check for an existing pointer in session fixes this.

Call-outs:

N/A

Testing:

New check inserted in an external session cache check.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.75%. Comparing base (0cfd3ed) to head (800e172).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1974      +/-   ##
==========================================
- Coverage   78.77%   78.75%   -0.02%     
==========================================
  Files         590      590              
  Lines      101482   101482              
  Branches    14396    14397       +1     
==========================================
- Hits        79945    79926      -19     
- Misses      20902    20920      +18     
- Partials      635      636       +1     

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