Bug 81859 points out that my fix for bug 81586 wasn't quite
right (or complete):  the argument of a %.*s directive need
not be a nul-terminated string when the precision is less
than the initialized size of the array the argument points
to.  The attached tweak uses strnlen to avoid reading past
the end of a non-nul terminated array.

The patch has been tested on x86_64-linux and by running
self tests under Valgrind.  I'll go ahead and commit it as
obvious sometime later today if there are no objections in
the meantime.

Martin