Server crashes when executing PS having ALTER statement after setting empty character_set_collations, UBSAN: member call on null pointer of type 'struct Reprepare_observer'

PREPARE s FROM "ALTER TABLE t modify b TEXT CHARACTER SET utf8 DEFAULT '' ";

SET @@character_set_collations='';

EXECUTE s;

Core was generated by `/test/MD270723-mariadb-11.2.0-linux-x86_64-opt/bin/mariadbd --no-defaults --cor'.

Program terminated with signal SIGSEGV, Segmentation fault.

#0  Reprepare_observer::report_error (this=0x0, thd=<optimized out>)

    at /test/11.2_opt/sql/sql_prepare.cc:3808

[Current thread is 1 (Thread 0x14f500088700 (LWP 3371857))]

(gdb) bt

#0  Reprepare_observer::report_error (this=0x0, thd=<optimized out>) at /test/11.2_opt/sql/sql_prepare.cc:3808

#1  0x0000560e602cf83b in Prepared_statement::check_charset_collation_map_version (this=0x14f4a4016948, this=0x14f4a4016948, observer=<optimized out>, thd=<optimized out>) at /test/11.2_opt/sql/sql_prepare.cc:238

#2  Prepared_statement::execute (this=0x14f4a4016948, expanded_query=0x14f500086e00, open_cursor=false) at /test/11.2_opt/sql/sql_prepare.cc:4898

#3  0x0000560e602cfb65 in Prepared_statement::execute_loop (this=this@entry=0x14f4a4016948, expanded_query=expanded_query@entry=0x14f500086e00, open_cursor=open_cursor@entry=false, packet=packet@entry=0x0, packet_end=packet_end@entry=0x0) at /test/11.2_opt/sql/sql_prepare.cc:4433

#4  0x0000560e602cfe7b in mysql_sql_stmt_execute (thd=thd@entry=0x14f4a4000c58) at /test/11.2_opt/sql/sql_prepare.cc:3470

#5  0x0000560e602af911 in mysql_execute_command (thd=0x14f4a4000c58, is_called_from_prepared_stmt=<optimized out>) at /test/11.2_opt/sql/sql_parse.cc:3960

#6  0x0000560e6029e1e5 in mysql_parse (rawbuf=<optimized out>, length=<optimized out>, parser_state=<optimized out>, thd=0x14f4a4000c58) at /test/11.2_opt/sql/sql_parse.cc:7800

#7  mysql_parse (thd=0x14f4a4000c58, rawbuf=<optimized out>, length=<optimized out>, parser_state=<optimized out>) at /test/11.2_opt/sql/sql_parse.cc:7722

#8  0x0000560e602aa092 in dispatch_command (command=COM_QUERY, thd=0x14f4a4000c58, packet=<optimized out>, packet_length=<optimized out>, blocking=<optimized out>) at /test/11.2_opt/sql/sql_class.h:1374

#9  0x0000560e602abf7e in do_command (thd=0x14f4a4000c58, blocking=blocking@entry=true) at /test/11.2_opt/sql/sql_parse.cc:1405

#10 0x0000560e603cb6cf in do_handle_one_connection (connect=<optimized out>, connect@entry=0x560e628f4b78, put_in_cache=put_in_cache@entry=true) at /test/11.2_opt/sql/sql_connect.cc:1445

#11 0x0000560e603cb9bd in handle_one_connection (arg=0x560e628f4b78) at /test/11.2_opt/sql/sql_connect.cc:1347

#12 0x000014f519530609 in start_thread (arg=<optimized out>) at pthread_create.c:477

#13 0x000014f51911c133 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95