style: fix declarations without initial value

[jmayclin]

Description of changes:

s2n-tls expects that all variable declarations set an initial value. This commit fixes violations of that rules across our codebase. This commit is an automated fix generated using clang-tidy.

The following script was used to generate the fixes

# remove the existing build directory
rm build -rf
# generate a compile command data base so clang-tidy knows the 
# structure of the project and it's included files
cmake . -Bbuild \
      -DCMAKE_C_COMPILER=/usr/bin/clang \
      -DCMAKE_BUILD_TYPE=RelWithDebInfo \
      -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
# run clang tidy, and have it fix the violations it can
run-clang-tidy . -p build/ -checks=cppcoreguidelines-init-variables -fix

More documentation on the init-variables check can be found here.

Call-outs:

I plan to add this as a check in the CI, but didn't want to add that into this PR so that this PR would include the minimum manual changes for it to pass CI.

Testing:

This is an automated fix with no expected behavior change. I have a murky recollection that most modern compilers will zero-init by default, but regardless all tests passing in CI should confirm that this is not a behavior change.

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

[jmayclin]

CppCheck is failing because of the following errors.

[tests/unit/s2n_self_talk_broken_pipe_test.c:95]: (style:unreadVariable) Variable 'pid' is assigned a value that is never used.
[tests/unit/s2n_self_talk_min_protocol_version_test.c:67]: (style:unreadVariable) Variable 'pid' is assigned a value that is never used.
[tests/unit/s2n_self_talk_tls12_test.c:105]: (style:unreadVariable) Variable 'pid' is assigned a value that is never used.
[tests/unit/s2n_tls12_handshake_test.c:495]: (style:unreadVariable) Variable 'handshake_type_original' is assigned a value that is never used.
[tests/unit/s2n_tls13_handshake_state_machine_test.c:659]: (style:unreadVariable) Variable 'handshake_type' is assigned a value that is never used.

These errors happen because we tend to push variables up to the next scope when dealing with loops. For example, this is s2n_self_talk_min_protocol_version_test.c

    s2n_blocked_status blocked;
    int status = 0;
    pid_t pid = 0;
    char cert_chain_pem[S2N_MAX_TEST_PEM_SIZE];
    char private_key_pem[S2N_MAX_TEST_PEM_SIZE];

    for (uint8_t version = S2N_TLS12; version <= S2N_TLS13; version++) {

        pid = fork();
        if (pid == 0) {

        }

Rather than writing a cppcheck suppression for this case, I think we should just move the pid_t declaration inside the for loop. If reviewers agree I'll go ahead and do that in a separate PR to keep this one entirely automated. On a side note, I'm guessing that this is only causing issues in this refactor because we currently suppress the variableScope warning.

s2n-tls/codebuild/bin/cppcheck_suppressions.txt

Lines 1 to 3 in 6c7cdc8

	// Message: (style:variableScope) The scope of the variable 'text' can be reduced.
	// Reason: Don't error for being able to reduce scope of variables in tests
	variableScope:tests/unit/*