MQTT contains many `check_pointer` calls without claiming them first

[resistor]

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.