Functions call each other, like

  nm_connection_get_id()
    nm_connection_get_setting_connection()
      nm_connection_get_setting()

Along the way, each function asserts that the input argument
is of type NMConnection via

    g_return_val_if_fail (NM_IS_CONNECTION (connection), NULL);

Avoid such duplicate assertions when we already verifyied the
input argument.

For example, in case of nm_connection_get_id(), don't check just call
nm_connection_get_setting_connection() right away. It already
asserts.

The downside is, that the assertion no longer fails in the function
that immediately called it. But these are assertions after all.