Incorrect loop conditional

I came across a problem while experimenting with MicroPython (v1.9.4). The program crashed while trying to execute this while loop:

        while (--d >= dig) {
            a = (a << DIG_SIZE) | *d;
            *d = a / base;
            a %= base;
        }

The pre-decrement (i.e. --) operation underflows the d pointer which caused the d >= dig condition to succeed (even though it shouldn't). Therefore, the line

*d = a / base

was mistakenly executed using an out-of-bounds pointer d.

Start in py/mpz.c at the loop around line 1705 and trace the bounds of d through the pre-decrement and comparison. Reproduce the MicroPython v1.9.4 crash described in the issue, then verify that the loop no longer dereferences an out-of-bounds pointer while preserving the intended calculation.