Fix one-byte buffer overrun in PQprintTuples().

This bug goes back to the original Postgres95 sources.  Its significance
to modern PG versions is marginal, since we have not used PQprintTuples()
internally in a very long time, and it doesn't seem to have ever been
documented either.  Still, it *is* exposed to client apps, so somebody
out there might possibly be using it.

Xi Wang