Fix: `tdi_low_reset` crash

Detailed description

In this PR we address a crash that was reported by triggered_tux on Discord and via the issue tracker.

In this crash, if the user has not yet done a JTAG scan on their probe or run any JTAG remote protocol commands since boot prior to issuing mon tdi_low_reset, then the probe (or BMDA if done in BMDA) will crash due to trying to call a NULL pointer in the jtag_proc structure. To rectify this, we guard this call and make the command fail if the JTAG structure has not been initialised yet.

Your checklist for this pull request

[x] I've read the Code of Conduct
[x] I've read the guidelines for contributing to this repository
[x] It builds for hardware native (see Building the firmware)
[x] It builds as BMDA (see Building the BMDA)
[x] I've tested it to the best of my ability
[x] My commit messages provide a useful short description of what the commits do

Closing issues

Fixes #1979

blackmagic-debug / blackmagic