Fix NULL jtagtap_cycle() which crashed BMDA

[ALTracer]

Detailed description

• No real new features.
• The existing problem is SIGSEGV in BMDA when trying to invoke jtag_proc.jtagtap_cycle() in initial JTAG scan.
• This PR solves it by providing simplified wrappers for that (around jtagtap_next()).

Affected backends are FTDI MPSSE, J-Link, and CMSIS-DAP.
Unaffected backends are BMP HL remote, STLINK. BMF was not affected either.
Tested with FT2232HL against a RV32I target, the other 3 callsites are not fixed.

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 (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• 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

[ALTracer]

Now that all three affected backends have got some implementations of that, commits 1-2 can be discarded (explicit NULL and a nullcheck at callsite).

[dragonmux]

This is looking better - we've got one review note that applies in a couple of places which is just about layout, otherwise LGTM 👍🏼

[dragonmux]

LGTM, merging. Thank you for the contribution! We'll backport this just like we did the JTAG scan fix as this fixes a v2.0 bug.