board disappears when in safe mode

[Neradoc]

Booting into safe mode the board does not appear on USB anymore (no MSC, no serial) until reset into normal mode.
Tested on macOS Catalina, but I think it's either a crash of the board or USB being disabled in safe mode.
Tested with a QT PY M0, Feather M4 pressing reset-to-boot, and a pico using the following code:

>>> import microcontroller
>>> microcontroller.on_next_reset(microcontroller.RunMode.SAFE_MODE)
>>> microcontroller.reset()

Tested a few builds, it seems to happen starting with #4689 (dynamic USB descriptors)

[dhalbert]

I know what the problem is and will fix it.