Fix memory leak in ztest

This is tripping LeakSanitizer, which causes zloop test failures on pull
requests.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14583

cmd/ztest.c

@@ -4639,8 +4639,11 @@ ztest_dmu_object_alloc_free(ztest_ds_t *zd, uint64_t id)
 	 * Destroy the previous batch of objects, create a new batch,
 	 * and do some I/O on the new objects.
 	 */
-	if (ztest_object_init(zd, od, size, B_TRUE) != 0)
+	if (ztest_object_init(zd, od, size, B_TRUE) != 0) {
+		zd->zd_od = NULL;
+		umem_free(od, size);
 		return;
+	}
 
 	while (ztest_random(4 * batchsize) != 0)
 		ztest_io(zd, od[ztest_random(batchsize)].od_object,