Fix MISRA C 2012 deviations

[chinglee-iot]

In this PR

• Suppress rule 18.2 deviations with comment. The pointer addresses elements of the same array.
• Fix rule 18.4 deviations. Not to apply operators to an expression of pointer type.
• Fix rule 21.2 deviations. Not to use reserved identifier in internal variables.

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

[AniruddhaKanhere]

We should also update the README here: https://github.com/aws/SigV4-for-AWS-IoT-embedded-sdk/blob/main/tools/coverity/README.md to say the actual version being used. Currently is says that:

Note For generating the report as outlined below, we have used Coverity version 2018.09.

[archigup]

Approved, given we make the changes to not need the exception afterwards.