MINOR: backend: switch to conn_get_{src,dst}() for port and address mapping

The backend connect code uses conn_get_{from,to}_addr to forward addresses
in transparent mode and to map server ports, without really checking if the
operation succeeds. In preparation of future changes, let's switch to
conn_get_{src,dst}() and integrate status check for possible failures.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 6727921..e0f6991 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -526,7 +526,7 @@
 
 	/* needs src ip/port for logging */
 	if (si->flags & SI_FL_SRC_ADDR)
-		conn_get_from_addr(conn);
+		conn_get_src(conn);
 
 	return ret;
 }