MDEV-26561 mariabackup release locks

The previous threads locked need to be released too.

This occurs if the initialization of any of the non-first
mutex/conditition variables errors occurs.

extra/mariabackup/ds_compress.cc

@@ -386,6 +386,13 @@ create_worker_threads(uint n)
 	return threads;
 
 err:
+	while (i > 0) {
+		comp_thread_ctxt_t *thd;
+		i--;
+		thd = threads + i;
+		pthread_mutex_unlock(&thd->ctrl_mutex);
+	}
+
 	my_free(threads);
 	return NULL;
 }