Fix digit-formatting calculation for small numbers.

Fix digit-formatting calculation for small numbers.

Calling log10() on zero causes a non-sensical value to be calculated. On some
compile options, I've observed yosys crashing with an illegal
instruction (SIGILL).

To make it safe, fix the calculation to do a range check; wrap it a
decimal_digits() function, and use it where the previous ceil(log10(n)) call
was used. As a side, it also improves readability.

Signed-off-by: Henner Zeller <h.zeller@acm.org>