Leak sanitizer error in File_query_log::set_rotated_name(bool)

Can be reproduced with "mtr --sanitize percona_slowlog_size_limits"

==2340519==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4044 byte(s) in 31 object(s) allocated from:
   #0 0x7f7ffd59936f in __interceptor_malloc (/lib64/libasan.so.6+0xab36f)
   #1 0x20537ce in my_raw_malloc /home/shurik/workspace/percona-server/5.7/mysys/my_malloc.c:203
   #2 0x2053ba4 in my_malloc /home/shurik/workspace/percona-server/5.7/mysys/my_malloc.c:66
   #3 0x205420f in my_strdup /home/shurik/workspace/percona-server/5.7/mysys/my_malloc.c:321
   #4 0x177b981 in File_query_log::set_rotated_name(bool) /home/shurik/workspace/percona-server/5.7/sql/log.cc:2285
   #5 0x177c075 in File_query_log::rotate(unsigned long) /home/shurik/workspace/percona-server/5.7/sql/log.cc:2305
   #6 0x177c487 in File_query_log::write_slow(THD*, unsigned long long, unsigned long long, char const*, unsigned long, unsigned long long, unsigned long long, bool, char const*, unsigned long) /home/shurik/wo
rkspace/percona-server/5.7/sql/log.cc:878
   #7 0x177d3e5 in Log_to_file_event_handler::log_slow(THD*, unsigned long long, unsigned long long, char const*, unsigned long, unsigned long long, unsigned long long, bool, char const*, unsigned long) /home/
shurik/workspace/percona-server/5.7/sql/log.cc:1432
   #8 0x1776490 in Query_logger::slow_log_write(THD*, char const*, unsigned long) /home/shurik/workspace/percona-server/5.7/sql/log.cc:1532
   #9 0x1777830 in log_slow_do(THD*) /home/shurik/workspace/percona-server/5.7/sql/log.cc:1998
   #10 0x1778bfa in log_slow_statement(THD*) /home/shurik/workspace/percona-server/5.7/sql/log.cc:2005
   #11 0x1960006 in dispatch_command(THD*, COM_DATA const*, enum_server_command) /home/shurik/workspace/percona-server/5.7/sql/sql_parse.cc:1996
   #12 0x196386c in do_command(THD*) /home/shurik/workspace/percona-server/5.7/sql/sql_parse.cc:1065
   #13 0x1b8240d in handle_connection /home/shurik/workspace/percona-server/5.7/sql/conn_handler/connection_handler_per_thread.cc:325
   #14 0x209f3a6 in pfs_spawn_thread /home/shurik/workspace/percona-server/5.7/storage/perfschema/pfs.cc:2198
   #15 0x7f7ffd4803f8 in start_thread (/lib64/libpthread.so.0+0x93f8)