Limit innodb_encryption_threads to 255

SET @@GLOBAL.innodb_encryption_threads=10000;  # Will make server + CLI hang AND stop any new clients from connecting

Core was generated by `/test/MD110420-mariadb-10.5.3-linux-x86_64-dbg/bin/mysqld --no-defaults --core-'.

Program terminated with signal SIGABRT, Aborted.

#0  __pthread_kill (threadid=<optimized out>, signo=signo@entry=6)

    at ../sysdeps/unix/sysv/linux/pthread_kill.c:57

[Current thread is 1 (Thread 0x7f507c15f700 (LWP 30081))]

(gdb) bt

#0  __pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ../sysdeps/unix/sysv/linux/pthread_kill.c:57

#1  0x0000562a6133b21e in my_write_core (sig=sig@entry=6) at /test/10.5_dbg/mysys/stacktrace.c:518

#2  0x0000562a60ae108f in handle_fatal_signal (sig=6) at /test/10.5_dbg/sql/signal_handler.cc:329

#3  <signal handler called>

#4  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51

#5  0x00007f507a8a3801 in __GI_abort () at abort.c:79

#6  0x0000562a610f73e8 in ut_dbg_assertion_failed (expr=expr@entry=0x562a6174c238 "os_thread_count <= srv_max_n_threads", file=file@entry=0x562a6174c208 "/test/10.5_dbg/storage/innobase/os/os0thread.cc", line=line@entry=146) at /test/10.5_dbg/storage/innobase/ut/ut0dbg.cc:60

#7  0x0000562a60f79921 in os_thread_create_func (func=func@entry=0x562a61223333 <fil_crypt_thread(void*)>, arg=arg@entry=0x0, thread_id=thread_id@entry=0x7f507c15d8d8) at /test/10.5_dbg/storage/innobase/os/os0thread.cc:146

#8  0x0000562a612221c8 in fil_crypt_set_thread_cnt (new_cnt=10000) at /test/10.5_dbg/storage/innobase/fil/fil0crypt.cc:2133

#9  0x0000562a60e94a01 in innodb_encryption_threads_update (save=0x7f504e8743b0) at /test/10.5_dbg/storage/innobase/handler/ha_innodb.cc:18812

#10 0x0000562a60851585 in sys_var_pluginvar::global_update (this=0x7f5079f785f0, thd=0x7f504e815088, var=0x7f504e874390) at /test/10.5_dbg/sql/sql_plugin.cc:3584

#11 0x0000562a6074f9e2 in sys_var::update (this=0x7f5079f785f0, thd=0x7f504e815088, var=0x7f504e874390) at /test/10.5_dbg/sql/set_var.cc:207

#12 0x0000562a6074ff0b in set_var::update (this=<optimized out>, thd=<optimized out>) at /test/10.5_dbg/sql/set_var.cc:859

#13 0x0000562a607512c8 in sql_set_variables (thd=thd@entry=0x7f504e815088, var_list=var_list@entry=0x7f504e819fa0, free=free@entry=true) at /test/10.5_dbg/sql/set_var.cc:746

#14 0x0000562a6083a50c in mysql_execute_command (thd=thd@entry=0x7f504e815088) at /test/10.5_dbg/sql/sql_parse.cc:4976

#15 0x0000562a608449d1 in mysql_parse (thd=thd@entry=0x7f504e815088, rawbuf=<optimized out>, length=<optimized out>, parser_state=parser_state@entry=0x7f507c15e450, is_com_multi=is_com_multi@entry=false, is_next_command=is_next_command@entry=false) at /test/10.5_dbg/sql/sql_parse.cc:7953

#16 0x0000562a60830719 in dispatch_command (command=command@entry=COM_QUERY, thd=thd@entry=0x7f504e815088, packet=packet@entry=0x7f504e867089 "SET @@GLOBAL.innodb_encryption_threads=10000", packet_length=packet_length@entry=44, is_com_multi=is_com_multi@entry=false, is_next_command=is_next_command@entry=false) at /test/10.5_dbg/sql/sql_parse.cc:1839

#17 0x0000562a6082ef6f in do_command (thd=0x7f504e815088) at /test/10.5_dbg/sql/sql_parse.cc:1358

#18 0x0000562a60989a53 in do_handle_one_connection (connect=<optimized out>, connect@entry=0x7f5052c433a8, put_in_cache=put_in_cache@entry=true) at /test/10.5_dbg/sql/sql_connect.cc:1422

#19 0x0000562a60989d82 in handle_one_connection (arg=arg@entry=0x7f5052c433a8) at /test/10.5_dbg/sql/sql_connect.cc:1319

#20 0x0000562a60dea080 in pfs_spawn_thread (arg=0x7f5079c45888) at /test/10.5_dbg/storage/perfschema/pfs.cc:2201

#21 0x00007f507b5866db in start_thread (arg=0x7f507c15f700) at pthread_create.c:463

#22 0x00007f507a98488f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95