build: add lint for null checks

[jmayclin]

Description of changes:

s2n-tls has strict conventions for checking the nullness of pointers returned from fallible methods. See comment here: #4369 (comment). We rely on developers and reviewers to remember to correctly check all of these functions.

Rather than having this be a manual check, we can automate it's enforcement. This PR adds a simple python script that will automatically error when there is a place where nullness isn't checked.

Example output from the tool

File: ./tests/unit/s2n_send_multirecord_test.c:148: nullness of conn was not checked
    148 DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(S2N_CLIENT),
    149 s2n_connection_ptr_free);
    150 EXPECT_SUCCESS(s2n_connection_set_config(conn, config));
    151
    152 /* Uninitialized buffer */
You should add `EXPECT_NOT_NULL(conn);` after the call to s2n_connection_new

File: ./tests/unit/s2n_ecdsa_test.c:227: nullness of config was not checked
    227 DEFER_CLEANUP(struct s2n_config *config = s2n_config_new(),
    228 s2n_config_ptr_free);
    229 EXPECT_SUCCESS(s2n_config_set_cipher_preferences(config, "test_all"));
    230 EXPECT_SUCCESS(s2n_config_add_cert_chain_and_key_to_store(config, chain_and_key));
    231 EXPECT_SUCCESS(s2n_config_set_verification_ca_location(config,
You should add `EXPECT_NOT_NULL(config);` after the call to s2n_config_new

File: ./tests/unit/s2n_ecdsa_test.c:236: nullness of client was not checked
    236 DEFER_CLEANUP(struct s2n_connection *client = s2n_connection_new(S2N_CLIENT),
    237 s2n_connection_ptr_free);
    238 EXPECT_SUCCESS(s2n_connection_set_config(client, config));
    239
    240 DEFER_CLEANUP(struct s2n_connection *server = s2n_connection_new(S2N_SERVER),
You should add `EXPECT_NOT_NULL(client);` after the call to s2n_connection_new

This tool also includes an option to automatically --fix some of the violations. In the following PR I will apply the autofixes and also add a CI step enforcing this lint. In the meantime you can see the results here. 431 null checks were added.

Testing:

I applied the script and then manually spot checked some of the resulting files (the results can be seen in the linked branch). I also made sure that the resulting build compiled and passed all tests.

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

[dougch]

Neat!

[dougch]

Future issue/pr?

[jmayclin]

Some concerns have been raised that this PR is not a beneficial direction to move in, so hold off on merging this PR until s2n-tls maintainers have consensus on that.