Fix: unexpected E02 from gdb m-packets

[ALTracer]

Detailed description

• This is a minor but important bugfix.
• The existing problem is gdbserver returning unexpected read errors E02 Cannot access memory at address to otherwise normal accesses.
• The PR solves it by fixing a mistranslated bounds-check introduced in ca. v1.10.0-1567-g de64559 of Feature/gdb packet optimization #2014.

Tested on gd32f103cb bluepillplus running bluepill.ini/stlink BMF; and BMDA; against stm32g474ce. At first I expected wrong RCC/Flash config or invalid SRAM mapping, but interactive debugging of BMDA and extended logging proved otherwise.
Note: (gdb) maintenance packet m08070000,201 triggers a 513 byte read which is rejected by BMD. Only <=512 is good because of 1024 byte RSP packet buffer.

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

[dragonmux]

LGTM, nice work finding this bug - merging. Thank you for the contribution!

[perigoso]

Thanks for the cleanup, good catch!

I see this uses alloca a couple lines down, and I do see a way we can optimize that out!