auc: only add filesystem type if it is known

Prevent NULL-pointer on older versions of OpenWrt.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit f1969ab584)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

utils/auc/src/auc.c

@@ -1647,7 +1647,9 @@ int main(int args, char *argv[]) {
 	blobmsg_add_string(&reqbuf, "version", branch->version);
 	blobmsg_add_string(&reqbuf, "version_code", branch->version_code);
 	blobmsg_add_string(&reqbuf, "target", target);
-	blobmsg_add_string(&reqbuf, "filesystem", target_fstype?target_fstype:rootfs_type);
+
+	if (target_fstype || rootfs_type)
+		blobmsg_add_string(&reqbuf, "filesystem", target_fstype?target_fstype:rootfs_type);
 
 	sanetized_board_name = strdup(board_name);
 	tmp = sanetized_board_name;