shared/gatt-client: Fix not checking the return of bt_gatt_client_read_value

If the bt_gatt_client_read_value fails the whole discovery would stall
as ext_prop_read_cb would not be called.
diff --git a/src/shared/gatt-client.c b/src/shared/gatt-client.c
index 8fd8a45..4386692 100644
--- a/src/shared/gatt-client.c
+++ b/src/shared/gatt-client.c
@@ -631,9 +631,11 @@
 		return false;
 
 	handle = gatt_db_attribute_get_handle(attr);
-	bt_gatt_client_read_value(client, handle, ext_prop_read_cb,
+
+	if (!bt_gatt_client_read_value(client, handle, ext_prop_read_cb,
 							discovery_op_ref(op),
-							discovery_op_unref);
+							discovery_op_unref))
+		return false;
 
 	return true;
 }