Fix: Remote protocol target communications initialisation

[dragonmux]

Detailed description

This PR seeks to address a crash-inducing bug reported by ALTracer on Discord. Under certain conditions during failed communications with a target over the remote protocol, dp->error() is called and the remote protocol layer in the firmware was improperly initialising the faked DP structure it uses, resulting in an attempted call through a NULL function pointer.

This patch addresses this problem for both JTAG and SWD over remote protocol + documents that structure a bit to help prevent future problems.

Tested by ALTracer against the STM32MP157 with which this bug was found and triggered.

This bug has existed in the firmware since about 3 years ago and does affect v1.7, v1.8 and v1.9 - we may wish to backport it at least to v1.9 because of this.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[esden]

LGTM, also I think it is a good idea to backport this to the previous firmware branches.