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

contrib/pgbench/pgbench.c

@@ -1760,7 +1760,7 @@ parseQuery(Command *cmd, const char *raw_sql)
 	p = sql;
 	while ((p = strchr(p, ':')) != NULL)
 	{
-		char		var[12];
+		char		var[13];
 		char	   *name;
 		int			eaten;