MINOR: backend: Get client dst address to set the server's one only if needful

MINOR: backend: Get client dst address to set the server's one only if needful

In alloc_dst_address(), the client destination address must only be
retrieved when we are sure to use it. Most of time, this save a syscall to
getsockname(). It is not a bugfix in itself. But it revealed a bug in the
QUIC part. The CO_FL_ADDR_TO_SET flag is not set when the destination
address is create for anew quic client connection.