Lets imagine following call sequence:

connection_abort
threadpool_remove_connection
unlink_thd
thd_cleanup
THD::cleanup
close_temporary_tables
MYSQL_BIN_LOG::write
MYSQL_BIN_LOG::rotate
MYSQL_BIN_LOG::new_file_without_locking
MYSQL_BIN_LOG::new_file_impl
MYSQL_BIN_LOG::close
MYSQL_LOG::close
inline_mysql_file_sync
my_sync
before_sync_wait = tp_wait_begin
wait_begin

If there was only one client connection at the moment, assertion
(thread_group->connection_count > 0) will trigger.

In fact this happens constantly on rpl_rotate_logs.test in Percona Server 5.5
with threadpool. I cannot reproduce it in MariaDB 5.5 but after looking into
source code I think similar case might be possible.

Stack trace from PS:

Assertion failed: (thread_group->connection_count > 0), function wait_begin, file /Users/sergei/percona/repo/55-tp/Percona-Server-5.5.28-rel29.1/sql/threadpool_unix.cc, line 1150.

#0 0x00007fff9014fce2 in __pthread_kill ()
#1 0x00007fff85cac7d2 in pthread_kill ()
#2 0x00007fff85c9da7a in abort ()
#3 0x00007fff85cd05de in __assert_rtn ()
#4 0x00000001002d635c in wait_begin (thread_group=<value temporarily unavailable, due to optimizations>)
at threadpool_unix.cc:1150
#5 0x00000001002d63ac in tp_wait_begin (thd=0x101169600, type=6) at threadpool_unix.cc:1290
#6 0x00000001002f8337 in my_sync (fd=76, my_flags=108822512) at my_sync.c:60
#7 0x00000001000c0d7a in MYSQL_LOG::close (this=0x100a062c8, exiting=3) at mysql_file.h:1423
#8 0x00000001000c121b in MYSQL_BIN_LOG::close (this=0x100a062c0, exiting=3) at log.cc:6089
#9 0x00000001000c6d30 in MYSQL_BIN_LOG::new_file_impl (this=0x1067c8620, need_lock=false) at log.cc:4454
#10 0x00000001000c6f82 in MYSQL_BIN_LOG::new_file_without_locking ()
at /Users/sergei/percona/repo/55-tp/Percona-Server/sql/log.cc:4358
#11 0x00000001000c6f82 in MYSQL_BIN_LOG::rotate (this=0x100a062c0, check_purge=0x1067c86de, force_rotate=false)
at log.cc:5353
#12 0x00000001000c89a1 in MYSQL_BIN_LOG::write (this=0x1067c8910, event_info=0x1067c8910) at log.cc:5234
#13 0x0000000100165891 in close_temporary_tables (thd=0x1067c8c50) at sql_base.cc:1778
#14 0x0000000100183b16 in THD::cleanup (this=0x101169600) at sql_class.cc:1500
#15 0x00000001000f1aa2 in unlink_thd (thd=0x101169600) at mysqld.cc:2131
#16 0x00000001002a781c in threadpool_remove_connection (thd=0x101169600) at threadpool_common.cc:175
#17 0x00000001002d5a98 in connection_abort [inlined] ()
at /Users/sergei/percona/repo/55-tp/Percona-Server/sql/threadpool_unix.cc:1255
#18 0x00000001002d5a98 in handle_event [inlined] ()
at /Users/sergei/percona/repo/55-tp/Percona-Server/sql/threadpool_unix.cc:1452
#19 0x00000001002d5a98 in worker_main (param=0x1067c8ee0) at threadpool_unix.cc:1489
#20 0x0000000100465e24 in pfs_spawn_thread (arg=0x1061a1360) at pfs.cc:1015
#21 0x00007fff85caa8bf in _pthread_start ()
#22 0x00007fff85cadb75 in thread_start ()