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.