MQTT contains many `check_pointer` calls without claiming them first

The `CheriotHeapChecker` static analysis reports these as errors, but @davidchisnall points out that this code is a mostly intended to be a sandbox where we could just let things crash.

We do one of the following:
* add the proper claims where needed, if we want this code to be defensively safe.
* strip out the `check_pointer` calls to save on code size, if we want this code to just crash on error.

Start by locating the MQTT code's check_pointer calls and reviewing the CheriotHeapChecker errors they produce. Determine with maintainers whether this sandbox should add claims or remove the checks; done means the chosen policy is applied consistently and the static analysis no longer reports these errors.

Written by the indexing model from the issue text.