this zeros-watchdog was caused by a SPI DMA error on STM32F405:
https://discuss.ardupilot.org/t/crash-with-4-2-0-beta-and-4-3-0-daily-bdshot/83297
we had incorrectly left these internal errors enabled when asserts
were not enabled. That led to a osalSysHalt()
without these we get an spi_fail internal error, caught by the
SPIDevice code