pr: fix -n to pad consistently and not crash

pr: fix -n to pad consistently and not crash

* src/pr.c: Replace the code to truncate the most significant
digits of line numbers, with much simpler string manipulation
that supports arbitrary widths.  Before this, specifying a
width >= 32 to -n would result in a divide by zero error.
Also remove the inconsistent padding with zeros and spaces, which
would result in zero padding for widths 12 and 15.
* tests/pr/pr-tests.pl: Added a test to ensure no zero padding,
and also a test for the divide by zero case.
* NEWS: Mentioned the fix
Reported by Ondrej Oprala