Fix GCC 7 snprintf() compiler warning.
Make buffer 1 byte larger to fit a sign. It's actually impossible for
there to be a sign in practice, but this is still required to keep GCC 7
happy.
Cleanup from commit
51bc271790eb234a1ba4d14d3e6530f70de92ab5.
Based on a suggestion from Peter Eisentraut.
Author: Peter Geoghegan
Reported-By: Peter Eisentraut
Discussion: https://postgr.es/m/
d1cc82ed-d07d-cef2-7c00-
2e987f121648@2ndquadrant.com