ipaccess: Handle the case of replacing RSL connections

Fix a infinite loop when establishing a new RSL connection and the
BSC is identifying itself with a unit id of an already established
RSL connection. The infinite loops happens because we are corrupting
the the linuxlist inside the bsc_fd when registering the bfd twice.

Due the lack of proper authentication favor the new RSL connection
as the real one and close the previous one.
diff --git a/openbsc/src/input/ipaccess.c b/openbsc/src/input/ipaccess.c
index 943a5e8..71a3bde 100644
--- a/openbsc/src/input/ipaccess.c
+++ b/openbsc/src/input/ipaccess.c
@@ -247,6 +247,13 @@
 			trx->rsl_link = e1inp_sign_link_create(e1i_ts,
 							E1INP_SIGN_RSL, trx,
 							trx->rsl_tei, 0);
+
+			if (newbfd->fd >= 0) {
+				LOGP(DINP, LOGL_ERROR, "BTS is still registered. Closing old connection.\n");
+				bsc_unregister_fd(newbfd);
+				close(newbfd->fd);
+			}
+
 			/* get rid of our old temporary bfd */
 			memcpy(newbfd, bfd, sizeof(*newbfd));
 			newbfd->priv_nr = 2+trx_id;