chan_sip: Fix port parsing for IPv6 addresses in SIP Via headers.

If a Via header containes an IPv6 address and a port number is ommitted,
as it is the standard port, we now leave the port empty and to not set it
to the value after the first colon of the IPv6 address.

ASTERISK-25443 #close

Change-Id: Ie3c2f05471cd006bf04ed15598589c09577b1e70