Fix for BUG#3017

"wrong Relay_Log_Pos if Rotate is in the middle of a transaction in relay log"
increment 'pending' instead of 'relay_log_pos'.

mysql-test/r/rpl_trunc_binlog.result

@@ -10,4 +10,4 @@ reset slave;
 start slave;
 show slave status;
 Master_Host	Master_User	Master_Port	Connect_retry	Master_Log_File	Read_Master_Log_Pos	Relay_Log_File	Relay_Log_Pos	Relay_Master_Log_File	Slave_IO_Running	Slave_SQL_Running	Replicate_do_db	Replicate_ignore_db	Last_errno	Last_error	Skip_counter	Exec_master_log_pos	Relay_log_space
-127.0.0.1	root	MASTER_PORT	1	master-bin.002	4	slave-relay-bin.002	161	master-bin.001	Yes	No			0	Rolling back unfinished transaction (no COMMIT or ROLLBACK) from relay log. Probably cause is that the master died while writing the transaction to it's binary log.	0	79	#
+127.0.0.1	root	MASTER_PORT	1	master-bin.002	4	slave-relay-bin.002	120	master-bin.001	Yes	No			0	Rolling back unfinished transaction (no COMMIT or ROLLBACK) from relay log. A probable cause is that the master died while writing the transaction to its binary log.	0	79	#

sql/log_event.cc

@@ -2121,7 +2121,7 @@ int Start_log_event::exec_event(struct st_relay_log_info* rli)
       slave_print_error(rli, 0,
                         "\
 Rolling back unfinished transaction (no COMMIT or ROLLBACK) from relay log. \
-Probably cause is that the master died while writing the transaction to it's \
+A probable cause is that the master died while writing the transaction to its \
 binary log.");
       return(1);
     }
@@ -2221,13 +2221,15 @@ int Rotate_log_event::exec_event(struct st_relay_log_info* rli)
     In that case, we don't want to touch the coordinates which correspond to the
     beginning of the transaction.
   */
-  if (!rli->inside_transaction)
+  if (rli->inside_transaction)
+    rli->inc_pending(get_event_len());
+  else
   {
     memcpy(rli->master_log_name, new_log_ident, ident_len+1);
     rli->master_log_pos= pos;
+    rli->relay_log_pos += get_event_len();
     DBUG_PRINT("info", ("master_log_pos: %lu", (ulong) rli->master_log_pos));
   }
-  rli->relay_log_pos += get_event_len();
   pthread_mutex_unlock(&rli->data_lock);
   pthread_cond_broadcast(&rli->data_cond);
   flush_relay_log_info(rli);