MDEV-7364 - mysqld --help --verbose prints random values for "debug"

getopt value pointer of "debug" variable was pointing to incorrect address:
&global_system_variables. Runtime statements like SHOW VARIABLES materialize
value from DBUG structures on demand, so they never access getopt value pointer.
But mysqld --help --verbose loaded this value from &global_system_variables.

Remove "debug" variable from mysqld --help --verbose output by setting value
pointer to NULL.

sql/sys_vars.h

@@ -741,7 +741,8 @@ class Sys_var_dbug: public sys_var
                on_check_function on_check_func=0,
                on_update_function on_update_func=0,
                const char *substitute=0)
-    : sys_var(&all_sys_vars, name_arg, comment, flag_args, 0, getopt.id,
+    : sys_var(&all_sys_vars, name_arg, comment, flag_args,
+              - (ptrdiff_t) &global_system_variables, getopt.id,
               getopt.arg_type, SHOW_CHAR, (intptr)def_val,
               lock, binlog_status_arg, on_check_func, on_update_func,
               substitute)