abisip-find: check bsc_fd_register() result

Change-Id: I72d713725d287d32ec90506099751aeb9b15ef15
Fixes: Coverity CID 70462
diff --git a/openbsc/src/ipaccess/abisip-find.c b/openbsc/src/ipaccess/abisip-find.c
index 9fe7df6..5b36272 100644
--- a/openbsc/src/ipaccess/abisip-find.c
+++ b/openbsc/src/ipaccess/abisip-find.c
@@ -194,7 +194,11 @@
 		exit(1);
 	}
 
-	osmo_fd_register(&bfd);
+	rc = osmo_fd_register(&bfd);
+	if (rc < 0) {
+		fprintf(stderr, "Cannot register FD\n");
+		exit(1);
+	}
 
 	timer.cb = timer_cb;
 	timer.data = &bfd;