qemu-io: Assert failure on the fuzzed qcow2 image

'qemu-io -c write' failed on the fuzzed image with missed refcount tables:

Sequence:
 1. Unpack the attached archive, make a copy of test.img
 2. Put copy.img and backing_img.cow in the same directory
 3. Execute
   qemu-io copy.img -c 'write 2856960 208896'

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

qemu-io: block/qcow2-cluster.c:910: handle_copied: Assertion `*host_offset == 0 
|| offset_into_cluster(s, guest_offset) == offset_into_cluster(s, *host_offset)'
 failed.

qemu.git HEAD 2d591ce2aeebf