Check for a missing option value in parse_virtual_addr_network
before asserting on the NULL in tor_addr_parse_mask_ports.
This avoids crashing on torrc lines like Vi[rtualAddrNetworkIPv[4|6]]
when no value follows the option.
Bugfix on 0.2.3 (de4cc126cb on 24 November 2012), fixes#14142.
