Fix false positive static analysis issue

Fix false positive static analysis issue

Infer complains:
```
test_tls.c:172: error: Uninitialized Value
  `_.cert` is read without initialization.
  170.
  171.     T(sctx = SSL_CTX_new(TLS_server_method()));
  172.     T(SSL_CTX_use_certificate(sctx, ck.cert));
           ^
  173.     T(SSL_CTX_use_PrivateKey(sctx, ck.pkey));
  174.     T(SSL_CTX_check_private_key(sctx));

test_tls.c:173: error: Uninitialized Value
  `_.pkey` is read without initialization.
  171.     T(sctx = SSL_CTX_new(TLS_server_method()));
  172.     T(SSL_CTX_use_certificate(sctx, ck.cert));
  173.     T(SSL_CTX_use_PrivateKey(sctx, ck.pkey));
           ^
  174.     T(SSL_CTX_check_private_key(sctx));
  175.
```

Altough the complain is false positive, fixing it will allow for future
use of the tools in automated tests.