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.

src/bin/pgbench/pgbench.c

@@ -2231,7 +2231,7 @@ parseQuery(Command *cmd, const char *raw_sql)
 	p = sql;
 	while ((p = strchr(p, ':')) != NULL)
 	{
-		char		var[12];
+		char		var[13];
 		char	   *name;
 		int			eaten;