snmp_agent: Fix null pointer dereference

snmp_agent: Fix null pointer dereference

As `table` field in `data_definition_t` is just backreference,
then `snmp_agent_free_data()` should not alter parent structure.

So, table columns unregistering and freeing has moved to new
`snmp_agent_free_table_columns()` function.

That also fixes null pointer dereference.

Closes: #2338.