qemu-io: Failure on a qcow2 image with the fuzzed refcount table

'qemu-io -c write' and 'qemu-io -c aio_write' crashes on a qcow2 image with a fuzzed refcount table.

Sequence:
 1. Unpack the attached archive, make a copy of test.img
 2. Put copy.img and backing_img.file in the same directory
 3. Execute
    qemu-io copy.img -c write 279552 322560
                      or
   qemu-io copy.img -c aio_write 836608 166400

Result: qemu-io was killed by SIGIOT with the reason:

qemu-io: block/qcow2-cluster.c:1291: qcow2_alloc_cluster_offset: Assertion `*host_offset != 0' failed.

qemu.git HEAD 69f87f713069f1f