Since commit e1d19c90, buildfarm member gharial has been failing with
symptoms indicating that snprintf sometimes returns -1 for buffer
overrun, even though it passes the added configure check.  Some
google research suggests that this happens only in limited cases,
such as when the overrun happens partway through a %d item.  Adjust
the configure check to exercise it that way.  Since I'm now feeling
more paranoid than I was before, also make the test explicitly verify
that the buffer doesn't get physically overrun.