Bugfix/coap parser mem access bugs

[mjurczak]

[x] I confirm this contribution is my own and I agree to license it with Apache 2.0. [x] I confirm the moderators may change the PR before merging it in.

Summary of changes

Fixes for out-ouf-bounds memory access, memory leak and infinite loop condition in coap packets parser.

The fixed issues described in detail in the following threads: https://github.com/ARMmbed/mbed-os/issues/12957 https://github.com/ARMmbed/mbed-os/issues/12948 https://github.com/ARMmbed/mbed-os/issues/12930 https://github.com/ARMmbed/mbed-os/issues/12928 https://github.com/ARMmbed/mbed-os/issues/12927 https://github.com/ARMmbed/mbed-os/issues/12926 https://github.com/ARMmbed/mbed-os/issues/12925

Reserved CVE identifiers related to the resolved issues: CVE-2020-12886 CVE-2020-12883 CVE-2020-12884 CVE-2020-12885 CVE-2020-12887

[esajaa]

Hi @mjurczak,

I've been going through your pull request, and to me it seems like a good addition. I asked for a second opinion, but so far it's passing all the tests and nothing really seems out of place. I pushed some formatting fixes to the branch, hopefully that's okay to you.

I also noticed that you had few other branches in your fork. Is all the content here in this PR, or are there more pull requests incoming?

Best Regards, Esa

[mjurczak]

Hi @esajaa , Thanks for the formatting fixes. The content of the remaining branches in my fork is included in this single PR. I hope this is okay.

Kind regards, Maciej