paging: In case paging failed stop it everywhere

In case we can't page on a BTS then stop it everywhere. The
callers of paging_request assume that this is kind of an
atomic operation and we should help with that.
diff --git a/openbsc/src/libbsc/paging.c b/openbsc/src/libbsc/paging.c
index 8bc10e6..6b46c6d 100644
--- a/openbsc/src/libbsc/paging.c
+++ b/openbsc/src/libbsc/paging.c
@@ -341,8 +341,10 @@
 			break;
 
 		rc = paging_request_bts(bts, subscr, type, cbfn, data);
-		if (rc < 0)
+		if (rc < 0) {
+			paging_request_stop(NULL, subscr, NULL, NULL);
 			return rc;
+		}
 		num_pages += rc;
 	} while (1);