BUG#6987: Added note when stat is failing on relay log, added mutex for log rotatation.

sql/log.cc:
  Added a note when stat failed on log file
sql/slave.cc:
  Added mutex for relay log rotation

sql/log.cc

@@ -853,6 +853,8 @@ int MYSQL_LOG::purge_logs(const char *to_log,
            of space that deletion will free. In most cases,
            deletion won't work either, so it's not a problem.
         */
+	sql_print_information("Failed to execute my_stat on file '%s'",
+			      log_info.log_file_name);
         tmp= 0; 
       }
     }

sql/slave.cc

@@ -4462,6 +4462,7 @@ void rotate_relay_log(MASTER_INFO* mi)
   RELAY_LOG_INFO* rli= &mi->rli;
 
   lock_slave_threads(mi);
+  pthread_mutex_lock(&mi->data_lock);
   pthread_mutex_lock(&rli->data_lock);
   /* 
      We need to test inited because otherwise, new_file() will attempt to lock
@@ -4492,6 +4493,7 @@ void rotate_relay_log(MASTER_INFO* mi)
   rli->relay_log.harvest_bytes_written(&rli->log_space_total);
 end:
   pthread_mutex_unlock(&rli->data_lock);
+  pthread_mutex_unlock(&mi->data_lock);
   unlock_slave_threads(mi);
   DBUG_VOID_RETURN;
 }