pgsql: Fix GCC 7 snprintf() compiler warning.

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/2b3031559a607833e4e265bb9726d52f398661e0

Modified Files
--------------
src/test/modules/test_bloomfilter/test_bloomfilter.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)