Fix leak in bssl-compat SSL_get_servername()

[tedjpoole]

Fixes a leak that can occur if multiple calls are made to SSL_get_servername() from within the same certificate selection callback (see SSL_CTX_set_select_certificate_cb()).

[twghu]

The fix looks good :)

I understand the test was run under valgrind.

This might be a candidate pattern for future memory leak tests:

  TEST(SSLTest, test_SSL_get_servername_no_leak) {
    // Track memory before
    size_t mem_before = GetCurrentMemoryUsage();

    // Exercise the code
    for (int i = 0; i < 1000; i++) {
      SSL_get_servername(...);
    }

    // Force cleanup
    SSL_free(ssl);

    // Verify memory returned
    size_t mem_after = GetCurrentMemoryUsage();
    EXPECT_NEAR(mem_before, mem_after, tolerance);
  }

[twghu]

lgtm