Fix compiler warning

Fix compiler warning

With some newer gcc versions (8 and 9) you get a -Wformat-overflow
warning here.  In PG11 and later this was already fixed.  Since it's
trivial, backport it to get the older branches building without
warnings.

This back-patches commit 333a186dc into out-of-support branches,
pursuant to newly-established project policy.  The point is to
suppress scary-looking warnings so that people building these
branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com