BUG/MINOR: backend: fix target address retrieval in transparent mode

BUG/MINOR: backend: fix target address retrieval in transparent mode

A very old bug resulting from some code refactoring causes
assign_server_address() to refrain from retrieving the destination
address from the client-side connection when transparent mode is
enabled and we're connecting to a server which has address 0.0.0.0.

The impact is low since such configurations are unlikely to ever
be encountered. The fix should be backported to older branches.