circuit_send_intermediate_onion_skin() and extend_cell_format() should check for IPv6

When circuit_send_intermediate_onion_skin() and extend_cell_format() handle tor_addr_t, they assume they are IPv4.

But in #23502 (moved), we almost wrote code that sent them an IPv6 address. In this case, they put 0.0.0.0 in the extend cell, but they could issue a BUG() warning and refuse to send the cell instead.

Or they could send a proper IPv6 link specifier where the extend cell allows it.