BUG #14027: n_tup_ins increments regardless of insertion success

The following bug has been logged on the website:

Bug reference:      14027
Logged by:          Ilya Matveychikov
Email address:      matvejchikov@gmail.com
PostgreSQL version: 9.5.1
Operating system:   Linux
Description:        

postgres=# create table t (name text unique);
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
 n_tup_ins 
-----------
         0
postgres=# insert into t (name) values ('a');
INSERT 0 1
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
 n_tup_ins 
-----------
         1
postgres=# insert into t (name) values ('b');
INSERT 0 1
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
 n_tup_ins 
-----------
         2
postgres=# insert into t (name) values ('a');
ОШИБКА:  повторяющееся значение ключа нарушает ограничение уникальности
"t_name_key"
ПОДРОБНОСТИ:  Ключ "(name)=(a)" уже существует.
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
 n_tup_ins 
-----------
         3